Gold, Gold-silver Alloy Nanoparticle-functionalized PMMA Nanofibers for Ultrasensitive SERS-based Detection of Nile Blue and Malachite Green

Gold, Gold-silver Alloy Nanoparticle-functionalized PMMA Nanofibers for Ultrasensitive SERS-based Detection of Nile Blue and Malachite Green | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Gold, Gold-silver Alloy Nanoparticle-functionalized PMMA Nanofibers for Ultrasensitive SERS-based Detection of Nile Blue and Malachite Green Niharika Pradhan, Venugopal Rao Soma This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7436815/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 14 Oct, 2025 Read the published version in Plasmonics → Version 1 posted 15 You are reading this latest preprint version Abstract Flexile SERS substrates accompany distinct advantages for real-time detection, not only because of their high porosity, but also their ability to adapt to irregular surfaces and withstand bending or movement, making them ideal for practical, on-the-go sensing applications. The present work includes the fabrication of flexible substrates by combining electro-spun PMMA polymer nanofibers with femtosecond pulse laser ablated pure Gold (Au) and bimetallic (Au-Ag) NPs. Hydrophobic PMMA fibers were functionalised by drop casting plasmonic-active Au and Au-Ag NPs for surface-enhanced Raman scattering/spectroscopy (SERS) studies. A micro-Raman spectrometer (M/s Horiba) was used for the SERS data acquisition for two dye molecules, Nile blue (NB) and malachite green (MG). Au-loaded PMMA fibers detected up to 50 nM concentration of Nile blue and 200 nM concentration of MG, and also demonstrated a good reproducibility with R 2 values of 9.59% and 10.77% for NB (5 µM) and MG (5 µM), respectively. Similarly, PMMA-Au-Ag substrates detected NB with 10 nM concentration and Mg with 100 nM concentration as the lowest detection and showed reproducibility of 12.63% and 14.63% for NB (5 µM) and MG (5 µM), respectively. femtosecond laser ablation electrospinning surface-enhanced Raman spectroscopy PMMA-nanofibers Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Introduction Surface-enhanced Raman spectroscopy (SERS) has emerged as an effective technique for molecular fingerprinting [ 1 ]. This technique involves enhancing a low-intensity Raman signal in the vicinity of nano-entities by localised surface plasmon resonance (LSPR) [ 2 , 3 ]. LSPR concerns the collective oscillation of the free electron cloud confined in small metallic nano-entities at the metal-dielectric interfaces [ 2 ]. LSPR appears when the frequency of the incident source coincides with the oscillation frequency of these electrons, leading to a strong absorption and scattering of light and intensifying the local electric fields [ 2 – 4 ]. The enhancement of this localised field depends on various factors, like the size and shape of nanoparticles (NPs), the distance between two NPs, the surrounding dielectric medium, and the substrate chosen [ 4 – 7 ]. Despite its limitations, such as reproducibility, cost-effectiveness, its non-destructive data collection, and easy preparation make it a frontier method for various sensing applications [ 8 , 9 ]. In recent years, flexible SERS substrates have attracted enormous attention in sensing applications due to their low fabrication cost, simplicity in use, and real-time applications such as collecting analytes from uneven surfaces (e.g., bag, table) [ 10 , 11 ]. Owing to that, significant growth is perceived in this research area. Flexible SERS substrates such as cellulose, polymer films, and fabrics are available in both the research field and commercial applications [ 10 – 13 ]. Electrospinning has been well accepted as a highly efficient, cost-effective technique for producing continuous nanofibers from diverse materials, including polymers, composites, and ceramics, with a range of diameters [ 13 – 16 ]. Currently, nanofibers are being widely used in many areas, including dressing for wound care, drug delivery with controlled drug release, purification systems, biosensors, and SERS studies [ 13 ]. Even though rigid plasmonic SERS substrates are more proficient at trace-level detection, these flexible SERS substrates are more preferred due to their features, like high porosity and large surface area increase the loading of plasmonic active NPs in large quantities, ease in sample collection (e.g., from surfaces, suitcases, bags through swiping/swabbing), and low fabrication costs [ 13 , 17 , 18 ]. The performance of these flexible nanofiber substrates depends on some critical factors like the morphology and composition of the polymer fibers, as well as the characteristics of the nanoparticles, including their material, size, and distribution [ 4 – 6 , 19 ]. Furthermore, the decoration of nanoparticles onto the nanofibers plays another key role in determining the overall SERS performance [ 20 , 21 ]. The plain polymer nanofibers need to be functionalized by integrating with plasmonic active nanoparticles. Several approaches are there for synthesising plasmonic active nanofibers, including in situ preparation of metal NPs through chemical reduction, photoreduction of a metal in the polymer solution, a direct addition of pre-fabricated NPs into electrospinning polymer solution, and other post-spinning deposition (by coating, drop casting, vaporisation, etc.) or soaking of fibers into NPs colloidal solutions [ 13 , 19 , 22 – 24 ]. Bharathi et al. have synthesised PVA-Au nanofibers through electrospinning by laser-ablated Au NPs dissolved in PVA polymer solution [ 13 ]. Jalaja et al. coated electro-spun polystyrene nanofibers with a thin silver NPs layer and used that to detect RDX and DNT with an enhancement factor (EF) of order 10 5 and 10 6 , respectively [ 25 ]. Shue He et al. utilized gold nano stars (NS) loaded paper-based substrates for SERS study and detected crystal violet (CV) with an EF order of 10 7 [ 22 ]. Chao et al . worked on p-aminothiphenol detection with silver-deposited hydrophobic and superhydrophobic cotton SERS substrates and observed a better sensitivity than hydrophilic cotton SERS substrates due to the limitation of diffusion. They have reported an enhancement in Raman signal with an enhancement factor of 1.27×10 6 and 1.97×10 6 , for hydrophobic and superhydrophobic cotton SERS substrates, respectively [ 24 ]. Considering the post-deposition approaches, the performance of the final substrate is mainly dependent on the choice of NPs fabrication methods, particularly those offering high purity and good size control. In recent years, the technique of pulse laser ablation in liquid (PLAL) has gained significant approval for nanoparticle fabrication [ 13 , 26 , 27 ]. This one-step fabrication technique yields chemical contaminant-free, highly pure NPs through an intense pulse laser focusing on a bulk target [ 27 ]. The intense laser beam causes rapid vaporisation, followed by plasma bubble formation and bursting in a liquid atmosphere, which leads to melt nucleation and NPs creation. The input pulse duration is a critical parameter in laser-matter interaction. The parameters like fluence, wavelength, pulse duration, focal length, and repetition rate play significant roles in the ablation process [ 28 ]. Shorter pulse lasers correspond to minimal thermal destruction, hence cleaner and precise ablation in comparison to a longer pulse [ 29 ]. PLAL is now being used for a wide range of target materials, including metals, alloys, semiconductors, ceramics, etc [ 27 , 30 – 33 ]. This present work includes preparing hydrophobic flexible PMMA polymer nanofibers through electrospinning and functionalising them by drop casting femtosecond (fs) laser ablated Au and Au-Ag for SERS applications. These PMMA-Au and PMMA-Ag-Au flexible substrates were deployed for SERS analysis of Nile blue (NB) and Malachite Green (MG). Au and Au-Ag alloy (70%-30%) were selected for this SERS sensing based on strong support from prior well-optimised studies. Byram et al. have reported the successful laser ablation of pure Au and various Au–Ag alloy compositions, including the 70:30 Au-Ag ratio, and demonstrated their effectiveness in SERS applications [ 33 ]. Pure Au is highly reliable for SERS studies due to its high chemical stability and strong localised surface plasmon resonance (LSPR) properties. On the other side, while silver offers the highest sensitivity among plasmonic materials, its tendency towards rapid oxidation limits its adaptability to long-term sensing applications, hence, for real-time detection. The Au-Ag (70:30) alloy provides a balanced alternative, combining the stability of gold with the enhanced sensitivity of silver. Its higher Au percentage, i.e., 70%, can boost its life span in aqueous media, and can help in the SERS signal amplification in the case of flexible SERS substrates. Experimental Details Au and Au-Ag alloy NPs fabrication through femtosecond laser ablation: Pure Au and Au-Ag alloy NPs were fabricated through femtosecond laser ablation. A schematic of the typical experiment is presented in Fig. 1 . This process involves the interaction of a pulsed beam (~ 50 fs pulse width, 1000 Hz repetition rate, 800 nm central wavelength) with Au and Au-Ag alloy targets in a liquid medium. The femtosecond pulses used in this experiment were generated by a Ti: Sapphire laser system (Model: LIBRA, M/s Coherent, USA). Initially, the pure Au and Au-Ag alloy targets are ultrasonicated in acetone and water sequentially to ensure the removal of surface dopants. After cleaning, the targets are placed in Petri dishes, which were filled with 15 ml of distilled water (DW), and the Petri dishes were positioned on a motion-controlled stage one by one. The fs laser beam was focused using a focusing lens (focal length of ~ 15 cm) on the targets. Throughout the ablation experiment, the focused Gaussian beam raster scanned the targets at a speed of 0.1mm per second using that stage. The stage motion was monitored with a LabVIEW-interfaced motion controller (EPS 300, Newport, USA). An input pulse energy of 500 µJ was utilized in the experiments. As the raster scan commenced, a significant colour change was observed in the distilled water. After scanning, the colloidal solutions of the NPs were stored in clean glass vials. Flexible Substrate synthesis by the Electrospinning method: The flexible PMMA fiber substrate was produced by the electrospinning process [ 16 ]. This electrospinning technique is excellent for preparing free-standing long nanofibers from polymer melts. This experiment was carried out in an e-spin nano apparatus. The initial step is the preparation of the polymer solution, here a 10 wt.% PMMA homogenous polymer solution was prepared in dichloromethyl formamide (DMF) solvent through continuous stirring. The prepared solution was filled into a 5 ml syringe and placed inside the electrospinning chamber so that the distance between the syringe tip and drum collector was 15 cm. The syringe was connected to an anode, while the drum collector was connected to a cathode, and a high voltage of 16 kV was maintained between them. The polymer melts ejected from the syringe tip with a flow rate of 0.5 ml per hour. The experiment ran for 9 to 10 hours to produce a large area of nanofibrous sheet. We left the sheet for 1 hour to allow the solvent to evaporate. After it dried completely, the sheet was collected, as shown in Figs. 2 (b), 2(c). Figures 2 (d) and 2(e) show the pictures of PMMA nanofibers drop-casted with Au NPs and Au-Ag NPs, respectively. Figure 2 (f) shows the picture of dried nanofibers. SERS studies: Hydrophobic PMMA nanofibers were loaded with laser-ablated NPs by drop-casting and dried using a hot plate for the SERS analysis. The analytes Nile blue (NB) and Malachite green (MG) were drop-casted on the NPs integrated PMMA fibers, and the Raman spectra were recorded using a Horiba micro-Raman spectrometer, using a laser excitation source of 633 nm. The laser was focused onto the sample through an objective lens of 50X, providing a spot size of ~ 1.54 µm. To assess signal intensity and consistency, the spectra were collected from multiple randomly selected sites across the drop-cast area of each substrate. To prevent the material from photothermal damage, the laser power was maintained at 5% during data acquisition. The acquired data were plotted using Origin software to get the final SERS spectra, and the enhancement factors were calculated. Characterization: Primarily, the optical properties of the laser-ablated nanosolids were investigated through UV-visible absorption spectroscopy by using a JASCO V-750 spectrometer. The UV spectrum of Au nanoparticles displays a pronounced absorption peak at ~ 521 nm, confirming the presence of spherical nanoparticles. and for the Au-Ag solution, an absorption peak was observed at ~ 470 nm. The morphological and dimensional features of the NPs were extensively examined via transmission electron microscopy. These analyses were performed with a JEOL JEM-F200 transmission electron microscope (TEM), operated by a high-voltage source. This TEM study allowed for the exploration of the shape and size distribution of NPs. The surface structure of the nanofiber sheet was studied using field emission scanning electron microscopy (FESEM). The FESEM imaging was carried out using a Carl ZEISS Ultra 55 microscope. Through this high-resolution imaging, the surface morphology of the nanofibers and the distribution of nanoparticles on the surface were investigated. The static contact angle measurements were carried out to verify the surface wettability and hydrophobicity (contact angle greater than 90º). An ADCAM-02 apparatus was used to conduct this experiment, where a 2 µl distilled water droplet was dropped, and the contact angle measurements were carried out. The SERS studies of two dye molecules, NB and MG, were implemented to evaluate the enhancing capability of Au and Au-Ag nanoparticle-loaded nanofibers. SERS spectra were acquired with a Horiba micro-Raman instrument, using a monochromatic laser source of 633 nm. The laser was focused via a 50X objective lens, providing localised Surface plasmon excitation and efficient Raman signal collection from the analyte molecule present on the NPs-decorated fibre substrates. Result and Discussion UV-Visible characterisation studies: The UV-visible absorption data of fs ablated pure Au NPs and Au-Ag alloy NPs are shown in Fig. 3 (a) and Fig. 3 (b), respectively. This absorption in metal NPs corresponds to surface plasmon resonance (SPR). SPR results from the collective oscillations of loosely bound surface electrons in metals when illuminated with appropriate photon sources. Similarly, in the case of semiconductors, UV absorption results from the valence band to the conduction band transition of electrons. In the case of pure Au NPs, the SPR absorption peak was found at ~ 521 nm. Likewise, for Au − Ag NPs, a single SPR peak was observed at ~ 470 nm, signifying the formation of Au-Ag NPs. TEM studies: The structural features, such as shape, size, and interplanar spacing, were obtained from TEM imaging of NPs. The ablated colloidal NPs were drops cast onto the carbon-coated copper grids for TEM analysis and loaded into the apparatus. the loaded grids were exposed to a high-energy electron beam for data acquisition. Figure 4 and Fig. 5 illustrate the TEM images and detailed analysis of the ablated Au NPs and Au-Ag NPs, respectively. Figures 4 (a)-(c) and 5(a)-(c) illustrate the TEM images of Au NPs and Au-Ag NPs, respectively, at different magnifications. These TEM images were subsequently analysed through ImageJ software to evaluate particle size distribution, and the corresponding histogram plots are insets in Figs. 4 (a) and 5(a). Energy Dispersive X-ray (EDX) elemental mapping of Au nanocolloids and mass percentage are shown in Figs. 4 (d) and 4(e), respectively. Similarly, Fig. 5 (d) represents the EDX data of the Au-Ag nanocolloids, and Fig. 5 (e) illustrates the elemental mass percentage of Au-Ag nanocolloids. EDX elemental mapping data showed that the Au nano colloids mainly consist of 93% of Au and only 6% of oxygen by mass. This low oxygen content suggests limited surface oxidation, pointing to its superior chemical stability in aqueous media. In comparison, the gold-silver alloy nanoparticles contained 28% gold, 22% silver, and a significantly higher oxygen content of 49%. The raised oxygen level in the alloy sample indicates greater surface oxidation, reflecting lower stability in aqueous media. The average size measured corresponding to the Au NPs is ~ 17 nm, and the estimated sizes are ~ 17 nm and ~ 22 nm for Au-Ag NPs, as illustrated in Figs. 4 (a) and 5(a) in histogram plots. HRTEM images of Au and Au-Ag NPs are shown in Fig. 6 (a) and Fig. 7 (a), respectively. The IFFT images of the HRTEM images were obtained from ImageJ processing, and are shown in Fig. 6 (b) and Fig. 6 (c), depicting the d-spacing profile corresponding to it. Similarly, Figs. 7 (b) and 7(c) display the IFFT images obtained from the HRTEM images and the d-spacing profile of Au-Ag NPs, respectively. The interplanar spacing estimated for Au NPs was 0.250 nm, and for Au-Ag NPs it was 0.252 nm. Bharathi et al . reported an average size of ~ 15 nm and an interplanar spacing of 0.230 nm for picosecond laser-ablated Au NPs [ 21 ]. Kuliliute et al. recently synthesised Au NPs through femtosecond laser ablation in aqueous medium and found spherical particles, with a size distribution ranging from 4 nm to 50 nm [ 30 ]. Similarly, Hidayah et al. first ablated pure Au and Ag metal targets using a femtosecond laser in aqueous media, mixed the Au and Ag colloidal solution in a 1:1 ratio, and irradiated that mixture solution with a femtosecond laser beam. They reported an average diameter size of 15.03 nm Au-Ag (1:1) nanoparticles, synthesised through this irradiation [ 31 ]. SEM & contact angle studies: The FESEM images of plain PMMA fibers at different magnifications are shown in Fig. 8 (a) and 8(b). The inset histogram plot presented in Fig. 8 (a) demonstrates the fibre diameter distribution of these PMMA fibers obtained using ImageJ processing of the FESEM Images. The average fibre diameter was found to be ~ 642 nm. When compared to previous reports, Piperno et. al reported diameters ranging from 150 nm to 800 nm for electropolished PMMA fibers. Figures 8 (c) and 8(d) illustrate the FESEM images of the Au NPs dropped on PMMA fibers. Simultaneously, Figs. 8 (e) and 8(f) illustrate the FESEM images of Au-Ag alloy NPs dropped on PMMA nanofibers. The higher magnification image in 8(d) illustrates a good quantity of adsorbed Au NPs on the PMMA fiber surface, which can aid in a superior SERS enhancement. Since this is a simple dropping technique (easier for the user to perform this step), the deposition of the NPs onto the nanofibers may not be extremely uniform. In the future, we will consider spin coating the NPs using different speeds. This surface adsorption of these drop-cast colloidal NPs showcases the hydrophobic behaviour of nanofibers. Further, the hydrophobic behaviour of PMMA nanofibers was verified through contact angle measurement with a 2 ml DW droplet. The acquired contact angle was ~ 125º at the moment of drop casting, and it decreased to ~ 110º after 30 minutes, as shown in Figs. 9 (a) and 9(c), respectively. SERS studies: Rigid SERS substrates have already proven their efficiency in lower concentration analyte detection and reproducibility [ 34 – 38 ]. However, high manufacturing costs and difficulty in sample collection make these rigid substrates less preferable for real-time use. Considering this, cheaper, flexible SERS substrates have been extensively taken on for Sensing applications. Nanofibers are good options for flexible SERS substrates, facilitated by their high porosity, which enables a large surface area for nanoparticle loading and analyte adsorption. In our study, electrospinning fabricates flexible fibers used as substrates. These fibers were functionalized by drop casting plasmonic active Au NPs and Au-Ag NPs. These colloidal nanoparticles were deposited onto the hydrophobic fibers in varying volumes (1–50 µl). The NPs were drop-cast on the nanofibers and placed on a hot plate for drying. This approach enabled a systematic study on the effect of nanoparticle loading on SERS performance. The Raman measurements were carried two NB (5 µM) and MG (5 µM), for each volume, using both PMMA-Au and PMMA-Au-Ag flexible substrates. A gradual increase in the SERS intensity was noticed with an increase in the NPs loading. Figures 10 (a) and 10(b) showcase the related SERS plots using PMMA-Au substrates. Similarly, Figs. 11 (a) and 11(b) refer to PMMA-Au-Ag substrates. This occurrence of a saturated SERS signal can be attributed to the aggregation of nanoparticles, with an increase in the NPs volume. A saturation in the SERS intensities is observed beyond 20 µl in Figs. 10 (a) & 10(b). Similarly, Figs. 11 (a) and 11(b) show the SERS intensity saturation after 10 µl. Depending upon the volume of studies, the colloidal solution volume with better SERS intensity was finalized and used for concentration-dependent analysis. The concentration-dependent plots of the NB and MG with PMMA-Au substrates are shown in Figs. 10 (c) and 10(d). Similarly, Figs. 11 (c) and 11(d) correspond to the concentration-dependent plots of NB and MG dyes with PMMA-Au-Ag substrates. Our SERS studies revealed the observation of a superior enhancement with PMMA-Au-Ag substrates in the case of NB dye molecules. To examine the reproducibility of these flexible SERS substrates, the enhanced Raman signals were collected by focusing the laser source on different sites. The reproducibility of these flexible SERS substrates was estimated by evaluating the Relative Standard Deviation (RSD) among the SERS intensities corresponding to the most prominent peaks of the analyte. Figure 10 (e) [NB (5 µM) data] and 10(f) [MG (5 µM) data] depict the RSD data collected using PMMA-Au substrate, and Figs. 11 (e) [NB (5 µM) data] and 11(f)[ NB (5 µM) data] present the RSD data collected from PMMA-Au-Ag substrates. PMMA-Au substrate uniformity in SERS signal was observed across the drop-cast region with reproducibility of 9.59% and 10.77% for NB (5 µM) and MG (5 µM), respectively. In parallel, PMMA-Ag-Au substates also exhibited comparable SERS intensity with reproducibility of 12.63% and 14.63% for analyte NB (5 µM) and MG (5 µM), respectively. This notable uniformity in the SERS signal is closely associated with the structural advantage of the electrospun PMMA nanofibers. The dense, porous, and interconnected framework of nanofibers promoted the even distribution of NPs during drop-casting. On the other hand, the hydrophobic behaviour restricted the spreading of aqueous colloidal NPs, allowing the confined accumulation of NPs in a particular region [ 24 ]. This accumulation results in high NPs density, which leads to more SERS-active hotspots. The enhancement factors for both dyes are mentioned below in Table 1 . Figure 12 (a) and 12(b) depict the linear dependence of the SERS intensity versus concentration for the prominent peaks of MG (1616 cm − 1 ) and NB (592 cm − 1 ), respectively, obtained using the PMMA-Au substates. Similarly, Figs. 12 (c) and 12(d) depict the linear dependence of the peaks of MG (1616 cm − 1 ) and NB (592 cm − 1 ), respectively, achieved using the PMMA-Au-Ag substates. It is evident that there is a good linear dependence of the SERS intensity on the concentration of the analytes used. Table 1 The enhancement factors of dyes obtained using Au and Au-Ag NPs. Substrate Analyte Enhancement factor (Ef) PMMA drop-casted with Au Nano colloid Nile blue 2.27×10 4 Malachite green 1.03×10 4 PMMA drop-casted with Au-Ag Nano colloid Nile blue 2.93×10 5 Malachite green 4.41×10 4 Table 1 shows that PMMA-Au-Ag substrates demonstrated superior enhancement to PMMA-Au substrates. This refinement can be attributed to the combined plasmonic effects of Au and Ag, which create a more intense electromagnetic hotspot on the surface. The strong plasmonic response of Ag enhances the field strength, while Au provides stability, resulting in stronger Raman signals and improved detection at lower concentrations. The lowest detectable concentrations of the analyte were determined using the corresponding linear dependence of the SERS intensity versus concentration curves, applying the standard equation \(\:LOD=3.3\left(\sigma\:/S\right)\) . Here \(\:\sigma\:\) represents the standard deviation of the blank measurements, and \(\:S\) denotes the slope of the linear dependence curve [ 39 ]. The experimentally obtained LOD values, along with the corresponding theoretical predictions, are summarized in Table 2 below. Our future studies will involve collecting the analytes by swabbing/swiping techniques and performing the SERS measurements on different hazardous molecules. Table 2 Summary of the theoretical and experimental lowest detectable concentrations of analyte on NPs integrated PMMA nanofibers. Substrate Analyte Lowest concentration detected Limit of detection (LOD) PMMA drop-casted with Au Nanocolloids MG 200 nM 28 nM NB 50 nM 8 nM PMMA drop-casted with Au-Ag Nanocolloids MG 100 nM 20 nM NB 10 nM 2 nM Conclusions This work demonstrated the effective fabrication of Au and Au-Ag nanoparticles by focused femtosecond laser beam interaction with corresponding bulk material targets in an aqueous medium. The UV absorption spectra of these colloidal solutions depicted a peak absorbance at wavelengths ~ 522 nm and ~ 486 nm for Au and Au-Ag NPs, respectively. The single SPR peak in the Au-Ag colloidal solution confirms the presence of Au-Ag nanoparticles in solution. The TEM analysis revealed the average diameter distribution for Au NPs at ~ 17 nm and ~ 22 nm for Au-Ag NPs. The flexible PMMA nanofibrous sheet was prepared via the electrospinning technique and characterised through FESEM, before and after loading the NPs. The average fiber diameter was found to be ~ 662 nm. Further, these PMMA fibers were loaded with Au and Au-Ag NPs and used for SERS analysis. This PMMA-Au fiber NPs composite successfully detected NB at 50 nM and MG at 200 nM as the lowest possible concentration. Similarly, for the PMMA-Au-Ag composite, the lowest detected concentration was 10 nM for NB and 100 nM for MG. Further optimization is conceivable with different sizes of these nanoparticles and the diameters of the nanofibers, resulting in a variety of loading scenarios (i.e., different numbers of hotspots). We believe that these low-cost, easy-to-prepare nanofibers will be attractive in practical sensing applications, especially based on the SERS technique, achieving ppm and/or ppb levels. Declarations Funding V.R. Soma acknowledges the financial support from DRDO, India, through the DIA-CoE, ACRHEM. V.R. Soma also thanks the University of Hyderabad, India, for the IoE (Institute of Eminence) project [# UOH/IOE/RC1/RC1-20-016 ]. The IoE project was granted by the Ministry of Education, Government of India, vide MHRD, India notification F11/9/2019-U3(A). V.R. Soma also acknowledges the IOE, UoH, for funding through a collaborative Inter-Institutional Research Clusters Project UoH-IOE-IIRC-24-007 . 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Surf Interfaces 28:101616. https://doi.org/10.1016/j.surfin.2021.101616 Jalaja K, Bhuvaneswari S, Ganiga M, Divyamol R, Anup S, Cyriac J, George BK (2017) Effective SERS detection using a flexible wiping substrate based on electrospun polystyrene nanofibers. Anal Methods 9:3998–4003. https://doi.org/10.1039/c7ay00882a Zhang J, Claverie J, Chaker M, Ma D (2017) Colloidal Metal Nanoparticles Prepared by Laser Ablation and their Applications. ChemPhysChem 18:986–1006. https://doi.org/10.1002/cphc.201601220 Bharati MSS, Chandu B, Rao SV (2019) Explosives sensing using Ag–Cu alloy nanoparticles synthesized by femtosecond laser ablation and irradiation†. RSC Adv 9:1517–1525. https://doi.org/10.1039/c8ra08462a Attallah AH, Shamil F, Yasir A, Adawiya AA (2023) Effect of Liquid and Laser Parameters on Fabrication of Nanoparticles via Pulsed Laser Ablation in Liquid with Their Applications: A Review. Plasmonics 18:1307–1323. https://doi.org/10.1007/s11468-023-01852-7 Zayarny DA, Ionin AA, Kudryashov SI (2016) Pulse-width-dependent surface ablation of copper and silver by ultrashort laser pulses. Laser Phys Lett 13:076101. https://doi.org/10.1088/1612-2011/13/7/076101 Banerjee D, Akkanaboina M, Soma VR (2024) Plasmonic Nanoalloy Colloids Fabricated via Femtosecond Bessel Beam for Photonic and SERS-based Sensing Applications. Opt Mater 154:115668. https://doi.org/10.1016/j.optmat.2024.115668 Krishnakanth KN, Chandu B, Bharathi MSS, Santhosh S, Raavi K, Rao SV (2019) Ultrafast excited state dynamics and femtosecond nonlinear optical properties of laser fabricated Au and Ag 50 Au 50 nanoparticles. Opt Mater (Amst) 95:109239. https://doi.org/10.1016/j.optmat.2019.109239 Rawat R, Singh BK, Tiwari A, Arun N, Pathak AP, Shadangi Y, Mukhopadhyay NK, Nelamarri SR, Rao SV, Tripathi A (2022) Formation of CuNi enriched Phases During Laser Processing of Non-Equiatomic AlSiCrMnFeNiCu. J Alloys Compd 927:166905. https://doi.org/10.1016/j.jallcom.2022.166905 Byram C, Moram SSB, Banerjee D, Beeram R, Rathod J, Soma VR (2023) Review of ultrafast laser ablation for sensing and photonic applications. J Opt 25:043001. https://doi.org/10.1088/2040-8986/acbc31 Byram C, Soma VR (2017) 2,4-Dinitrotoluene Detected Using Portable Raman Spectrometer and Femtosecond Laser Fabricated Au-Ag Nanoparticles and Nanostructures, Nano-Structures and Nano-Objects. 12:121–129. https://doi.org/10.1016/j.nanoso.2017.09.019 Banerjee D, Akkanboina M, Ravi Kumar K, Ghose B, Soma VR (2024) Nanotubular Arrayed LIPSS Achieved With Femtosecond Bessel Beam on Silver Surface for SERS-based Sensing. J Phys Chem C 128:4655–4665. https://doi.org/10.1021/acs.jpcc.4c00160 Mangababu A, Banerjee D, Ravi Kumar K, Goud RSP, Soma VR, Nageswara Rao SVS (2023) Sub-70 nm Surface Structures on Femtosecond Laser Irradiated GaAs in Distilled Water and Sensing Application. Opt Lett 48(21):5539–5542. https://doi.org/10.1364/OL.502527 Rathod J, Bharati MSS, Byram C, Soma VR (2023) Single-step fabrication of hybrid germanium-gold/silver nanoentities by femtosecond laser ablation and applications in SERS-based sensing. Nanotechnology 34:405301. https://doi.org/10.1088/1361-6528/ace3c9 Beeram R, Soma VR (2023) Ultra-trace detection of multiple analytes using femtosecond laser structured Ag-Au alloy substrates and SERRS technique. Opt Mater 137:113615. https://doi.org/10.1016/j.optmat.2023.113615 Shrivastava A, Gupta V (2011) Methods for the determination of limit of detection and limit of quantitation of the analytical methods. Chronicles Young Sci 2:21. https://doi.org/10.4103/2229-5186.79345 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 14 Oct, 2025 Read the published version in Plasmonics → Version 1 posted Editorial decision: Revision requested 16 Sep, 2025 Reviews received at journal 16 Sep, 2025 Reviews received at journal 15 Sep, 2025 Reviewers agreed at journal 13 Sep, 2025 Reviews received at journal 12 Sep, 2025 Reviewers agreed at journal 09 Sep, 2025 Reviews received at journal 07 Sep, 2025 Reviewers agreed at journal 07 Sep, 2025 Reviewers agreed at journal 07 Sep, 2025 Reviewers agreed at journal 07 Sep, 2025 Reviewers agreed at journal 07 Sep, 2025 Reviewers invited by journal 07 Sep, 2025 Editor assigned by journal 25 Aug, 2025 Submission checks completed at journal 25 Aug, 2025 First submitted to journal 22 Aug, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7436815","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":513473354,"identity":"4184afa3-08e5-4ab9-8e98-76888cd691fa","order_by":0,"name":"Niharika Pradhan","email":"","orcid":"","institution":"University of Hyderabad","correspondingAuthor":false,"prefix":"","firstName":"Niharika","middleName":"","lastName":"Pradhan","suffix":""},{"id":513473359,"identity":"514a0f9e-e9f7-47a1-b3d5-77148a35850b","order_by":1,"name":"Venugopal Rao Soma","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA50lEQVRIiWNgGAWjYFAD9gYGZgYQAoIDxGnhOUCyFokEhBa8QL69O/EDY5tNnsHNN4aPC9usGfjbDzAeLsCjxeDM2c0SjG1pxQa3c4yNZ7alM0icSWA4PAOfFoncDRIMZw4nbridYybN23aYgeEGA8NhHnwOm5G7+QfDmf+JG26egWiRJ6SF4UbuNgmGigOJG27wQLQYENIC9Ms2C4aK5MSZZ9KKjXnOpfMYnklswO+w9t7NNxgM7BL7jh/e+JinzFpO7vjhw5/xOgwImP/AWIxsDEDFjA0ENKCAP4SVjIJRMApGwcgDAG2GTGQWMoWcAAAAAElFTkSuQmCC","orcid":"","institution":"University of Hyderabad","correspondingAuthor":true,"prefix":"","firstName":"Venugopal","middleName":"Rao","lastName":"Soma","suffix":""}],"badges":[],"createdAt":"2025-08-22 18:23:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7436815/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7436815/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11468-025-03298-5","type":"published","date":"2025-10-14T15:57:27+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":91199509,"identity":"db5f308e-2367-4856-aa41-504742793a19","added_by":"auto","created_at":"2025-09-12 15:20:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":109952,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic showing a detailed experimental set-up for femtosecond laser ablation.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/dfe1f77e59f17712a30696a6.png"},{"id":91199512,"identity":"da489958-b00a-4dee-8871-0fd336e793a6","added_by":"auto","created_at":"2025-09-12 15:20:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":200482,"visible":true,"origin":"","legend":"\u003cp\u003e(a) electrospinning apparatus (b) electrospun PMMA nanofiber sheet (c) PMMA nanofiber strip without NPs. PMMA nanofibers drop-casted with (d) Au NPs and (e) Au-Ag NPs, and (f) NPs drop-cast fibers after drying.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/e3fdae295df53738c4df043f.png"},{"id":91200588,"identity":"2f5e4548-046d-4f5d-af01-2f3fb81d1f8f","added_by":"auto","created_at":"2025-09-12 15:28:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":60097,"visible":true,"origin":"","legend":"\u003cp\u003eUV-Visible absorption spectra of (a) Au nanoparticles and (b) Au-Ag nanoparticles in DW using the laser ablation technique.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/87c693fdc47d9314436a2e6b.png"},{"id":91199511,"identity":"6a68aec6-2c34-47ab-b0ab-b4528d0b5847","added_by":"auto","created_at":"2025-09-12 15:20:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":170846,"visible":true,"origin":"","legend":"\u003cp\u003e(a) TEM image of fs laser ablated Au NPs in distilled water, and the inset histogram plot shows the particle size distribution of the NPs (b) and (c) TEM images of Au nano colloids with higher magnifications (d) elemental EDX data of Au nano colloids and (e) table showing elemental mass% obtained though EDX mapping.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/bd6ded2c0160f86a2cd22990.png"},{"id":91200936,"identity":"ed7fdab6-ac4e-40e3-b13b-01cd42484d6c","added_by":"auto","created_at":"2025-09-12 15:36:23","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":185078,"visible":true,"origin":"","legend":"\u003cp\u003e(a) TEM image of fs laser ablated Au-Ag NPs in distilled water, and the inset histogram plot shows the particle size distribution of the NPs (b) and (c) TEM images of Au-Ag NPs with higher magnifications (d) elemental EDX data of Au-Ag NPs and (e) table showing elemental mass% obtained through EDX mapping.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/7c3bcb7943932e5f8126cee4.png"},{"id":91199513,"identity":"8c22dc47-6d97-466c-8bac-8db109558a9a","added_by":"auto","created_at":"2025-09-12 15:20:23","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":331751,"visible":true,"origin":"","legend":"\u003cp\u003e(a) HRTEM image of fs laser ablated Au NPs in Distilled water, (b) IFFT processed image of HRTEM image of Au NPs, and (c) lattice spacing profile.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/394a6f86f3f8e0d8e11cf324.png"},{"id":91199517,"identity":"77a6dd4b-0948-4607-8ac9-9ac0454b0e14","added_by":"auto","created_at":"2025-09-12 15:20:24","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":285235,"visible":true,"origin":"","legend":"\u003cp\u003e(a) HRTEM image of fs laser ablated Au-Ag NPs in Distilled water, (b) IFFT processed image of HRTEM image of Au NPs, and (c) lattice spacing profile.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/d76dc487be11b145b9e222a4.png"},{"id":91200937,"identity":"acfa4a78-2c51-4b89-9afc-4a26c20608e6","added_by":"auto","created_at":"2025-09-12 15:36:24","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":259237,"visible":true,"origin":"","legend":"\u003cp\u003eFESEM images of plane PMMA fibers at (a) lower and (b) higher magnification with embedded histogram plot showing nanofiber diameter distribution (c) and (d) FESEM images of Au NPs loaded PMMA nanofibers, [(e) \u0026amp; (f)] FESEM images of Ag-Au NPs decorated nanofibers.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/8d558dec439d4c87fb1babcb.png"},{"id":91199523,"identity":"4694cdde-7151-432e-bcfa-68e528f8314e","added_by":"auto","created_at":"2025-09-12 15:20:24","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":76626,"visible":true,"origin":"","legend":"\u003cp\u003e(a) contact angle measured between a 2 µl water drop and plain PMMA nanofibers at the instant of drop casting (b) contact angle measured between a 2 µl water drop and plain PMMA nanofibers after 15 minutes of drop casting and (c) contact angle measured between a 2 µl water drop and plain PMMA nanofibers after 30 minutes of drop casting\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/7e7052862e1b8c8313c997d7.png"},{"id":91200938,"identity":"0243c8ab-a8a1-4298-a184-b654c7a798ae","added_by":"auto","created_at":"2025-09-12 15:36:24","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":119104,"visible":true,"origin":"","legend":"\u003cp\u003eThe SERS spectra of (a) NB (5 µM) and (b) MG (5 µM) acquired with different volumes of Au colloidal solution loaded PMMA nanofibers (c) concentration-dependent spectra of NB and (d) concentration-dependent spectra of MG (e) and (f) reproducibility data of PMMA-Au substrates for 590 cm\u003csup\u003e-1 \u003c/sup\u003eof NB (5 µM) and 1616 cm\u003csup\u003e-1 \u003c/sup\u003eof MG, respectively.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/119acfa5fa0df1e673aa86a2.png"},{"id":91200940,"identity":"31d99d2d-69d8-4c22-9eb6-4b65b1fac665","added_by":"auto","created_at":"2025-09-12 15:36:24","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":122851,"visible":true,"origin":"","legend":"\u003cp\u003eThe SERS spectra of (a) NB (5 µM) and (b) MG (5 µM) acquired with different volumes of Au-Ag colloidal solution loaded PMMA nanofibers (c) concentration-dependent spectra of NB and (d) concentration-dependent spectra of MG (e) and (f) reproducibility data of PMMA-Au-Ag substrates for 590 cm\u003csup\u003e-1 \u003c/sup\u003eof NB (5 µM) and 1616 cm\u003csup\u003e-1 \u003c/sup\u003eof MG, respectively.\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/e88950caab5d6088a9427468.png"},{"id":91199526,"identity":"e4ec3c69-bf73-4a35-8153-982e846d42d7","added_by":"auto","created_at":"2025-09-12 15:20:24","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":78266,"visible":true,"origin":"","legend":"\u003cp\u003eLinear dependence of the SERS intensity versus concentration for the prominent peaks of (a) MG and (b) NB using PMMA-Au substates and (c) MG and (d) NB using PMMA-Au-Ag substates.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/a5cdb8610861690f5a1527f4.png"},{"id":93955976,"identity":"45a7af38-29d1-4ad6-ae0a-8e117db11ee9","added_by":"auto","created_at":"2025-10-20 16:08:43","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2873623,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7436815/v1/b72ad551-4529-4127-a6f5-45b15e12a74d.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Gold, Gold-silver Alloy Nanoparticle-functionalized PMMA Nanofibers for Ultrasensitive SERS-based Detection of Nile Blue and Malachite Green","fulltext":[{"header":"Introduction","content":"\u003cp\u003eSurface-enhanced Raman spectroscopy (SERS) has emerged as an effective technique for molecular fingerprinting [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. This technique involves enhancing a low-intensity Raman signal in the vicinity of nano-entities by localised surface plasmon resonance (LSPR) [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. LSPR concerns the collective oscillation of the free electron cloud confined in small metallic nano-entities at the metal-dielectric interfaces [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. LSPR appears when the frequency of the incident source coincides with the oscillation frequency of these electrons, leading to a strong absorption and scattering of light and intensifying the local electric fields [\u003cspan additionalcitationids=\"CR3\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. The enhancement of this localised field depends on various factors, like the size and shape of nanoparticles (NPs), the distance between two NPs, the surrounding dielectric medium, and the substrate chosen [\u003cspan additionalcitationids=\"CR5 CR6\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Despite its limitations, such as reproducibility, cost-effectiveness, its non-destructive data collection, and easy preparation make it a frontier method for various sensing applications [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. In recent years, flexible SERS substrates have attracted enormous attention in sensing applications due to their low fabrication cost, simplicity in use, and real-time applications such as collecting analytes from uneven surfaces (e.g., bag, table) [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Owing to that, significant growth is perceived in this research area. Flexible SERS substrates such as cellulose, polymer films, and fabrics are available in both the research field and commercial applications [\u003cspan additionalcitationids=\"CR11 CR12\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Electrospinning has been well accepted as a highly efficient, cost-effective technique for producing continuous nanofibers from diverse materials, including polymers, composites, and ceramics, with a range of diameters [\u003cspan additionalcitationids=\"CR14 CR15\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Currently, nanofibers are being widely used in many areas, including dressing for wound care, drug delivery with controlled drug release, purification systems, biosensors, and SERS studies [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Even though rigid plasmonic SERS substrates are more proficient at trace-level detection, these flexible SERS substrates are more preferred due to their features, like high porosity and large surface area increase the loading of plasmonic active NPs in large quantities, ease in sample collection (e.g., from surfaces, suitcases, bags through swiping/swabbing), and low fabrication costs [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The performance of these flexible nanofiber substrates depends on some critical factors like the morphology and composition of the polymer fibers, as well as the characteristics of the nanoparticles, including their material, size, and distribution [\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Furthermore, the decoration of nanoparticles onto the nanofibers plays another key role in determining the overall SERS performance [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The plain polymer nanofibers need to be functionalized by integrating with plasmonic active nanoparticles. Several approaches are there for synthesising plasmonic active nanofibers, including in situ preparation of metal NPs through chemical reduction, photoreduction of a metal in the polymer solution, a direct addition of pre-fabricated NPs into electrospinning polymer solution, and other post-spinning deposition (by coating, drop casting, vaporisation, etc.) or soaking of fibers into NPs colloidal solutions [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan additionalcitationids=\"CR23\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Bharathi \u003cem\u003eet al.\u003c/em\u003e have synthesised PVA-Au nanofibers through electrospinning by laser-ablated Au NPs dissolved in PVA polymer solution [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Jalaja \u003cem\u003eet al.\u003c/em\u003e coated electro-spun polystyrene nanofibers with a thin silver NPs layer and used that to detect RDX and DNT with an enhancement factor (EF) of order 10\u003csup\u003e5\u003c/sup\u003e and 10\u003csup\u003e6\u003c/sup\u003e, respectively [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Shue He \u003cem\u003eet al.\u003c/em\u003e utilized gold nano stars (NS) loaded paper-based substrates for SERS study and detected crystal violet (CV) with an EF order of 10\u003csup\u003e7\u003c/sup\u003e [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Chao \u003cem\u003eet al\u003c/em\u003e. worked on p-aminothiphenol detection with silver-deposited hydrophobic and superhydrophobic cotton SERS substrates and observed a better sensitivity than hydrophilic cotton SERS substrates due to the limitation of diffusion. They have reported an enhancement in Raman signal with an enhancement factor of 1.27\u0026times;10\u003csup\u003e6\u003c/sup\u003e and 1.97\u0026times;10\u003csup\u003e6\u003c/sup\u003e, for hydrophobic and superhydrophobic cotton SERS substrates, respectively [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Considering the post-deposition approaches, the performance of the final substrate is mainly dependent on the choice of NPs fabrication methods, particularly those offering high purity and good size control. In recent years, the technique of pulse laser ablation in liquid (PLAL) has gained significant approval for nanoparticle fabrication [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. This one-step fabrication technique yields chemical contaminant-free, highly pure NPs through an intense pulse laser focusing on a bulk target [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. The intense laser beam causes rapid vaporisation, followed by plasma bubble formation and bursting in a liquid atmosphere, which leads to melt nucleation and NPs creation. The input pulse duration is a critical parameter in laser-matter interaction. The parameters like fluence, wavelength, pulse duration, focal length, and repetition rate play significant roles in the ablation process [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Shorter pulse lasers correspond to minimal thermal destruction, hence cleaner and precise ablation in comparison to a longer pulse [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. PLAL is now being used for a wide range of target materials, including metals, alloys, semiconductors, ceramics, etc [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan additionalcitationids=\"CR31 CR32\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThis present work includes preparing hydrophobic flexible PMMA polymer nanofibers through electrospinning and functionalising them by drop casting femtosecond (fs) laser ablated Au and Au-Ag for SERS applications. These PMMA-Au and PMMA-Ag-Au flexible substrates were deployed for SERS analysis of Nile blue (NB) and Malachite Green (MG). Au and Au-Ag alloy (70%-30%) were selected for this SERS sensing based on strong support from prior well-optimised studies. Byram et al. have reported the successful laser ablation of pure Au and various Au\u0026ndash;Ag alloy compositions, including the 70:30 Au-Ag ratio, and demonstrated their effectiveness in SERS applications [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Pure Au is highly reliable for SERS studies due to its high chemical stability and strong localised surface plasmon resonance (LSPR) properties. On the other side, while silver offers the highest sensitivity among plasmonic materials, its tendency towards rapid oxidation limits its adaptability to long-term sensing applications, hence, for real-time detection. The Au-Ag (70:30) alloy provides a balanced alternative, combining the stability of gold with the enhanced sensitivity of silver. Its higher Au percentage, i.e., 70%, can boost its life span in aqueous media, and can help in the SERS signal amplification in the case of flexible SERS substrates.\u003c/p\u003e"},{"header":"Experimental Details","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eAu and Au-Ag alloy NPs fabrication through femtosecond laser ablation:\u003c/h2\u003e\u003cp\u003ePure Au and Au-Ag alloy NPs were fabricated through femtosecond laser ablation. A schematic of the typical experiment is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. This process involves the interaction of a pulsed beam (~\u0026thinsp;50 fs pulse width, 1000 Hz repetition rate, 800 nm central wavelength) with Au and Au-Ag alloy targets in a liquid medium. The femtosecond pulses used in this experiment were generated by a Ti: Sapphire laser system (Model: LIBRA, M/s Coherent, USA). Initially, the pure Au and Au-Ag alloy targets are ultrasonicated in acetone and water sequentially to ensure the removal of surface dopants. After cleaning, the targets are placed in Petri dishes, which were filled with 15 ml of distilled water (DW), and the Petri dishes were positioned on a motion-controlled stage one by one. The fs laser beam was focused using a focusing lens (focal length of ~\u0026thinsp;15 cm) on the targets. Throughout the ablation experiment, the focused Gaussian beam raster scanned the targets at a speed of 0.1mm per second using that stage. The stage motion was monitored with a LabVIEW-interfaced motion controller (EPS 300, Newport, USA). An input pulse energy of 500 \u0026micro;J was utilized in the experiments. As the raster scan commenced, a significant colour change was observed in the distilled water. After scanning, the colloidal solutions of the NPs were stored in clean glass vials.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eFlexible Substrate synthesis by the Electrospinning method:\u003c/h3\u003e\n\u003cp\u003eThe flexible PMMA fiber substrate was produced by the electrospinning process [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. This electrospinning technique is excellent for preparing free-standing long nanofibers from polymer melts. This experiment was carried out in an e-spin nano apparatus. The initial step is the preparation of the polymer solution, here a 10 wt.% PMMA homogenous polymer solution was prepared in dichloromethyl formamide (DMF) solvent through continuous stirring. The prepared solution was filled into a 5 ml syringe and placed inside the electrospinning chamber so that the distance between the syringe tip and drum collector was 15 cm. The syringe was connected to an anode, while the drum collector was connected to a cathode, and a high voltage of 16 kV was maintained between them. The polymer melts ejected from the syringe tip with a flow rate of 0.5 ml per hour. The experiment ran for 9 to 10 hours to produce a large area of nanofibrous sheet. We left the sheet for 1 hour to allow the solvent to evaporate. After it dried completely, the sheet was collected, as shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b), 2(c). Figures\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d) and 2(e) show the pictures of PMMA nanofibers drop-casted with Au NPs and Au-Ag NPs, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(f) shows the picture of dried nanofibers.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eSERS studies:\u003c/h3\u003e\n\u003cp\u003eHydrophobic PMMA nanofibers were loaded with laser-ablated NPs by drop-casting and dried using a hot plate for the SERS analysis. The analytes Nile blue (NB) and Malachite green (MG) were drop-casted on the NPs integrated PMMA fibers, and the Raman spectra were recorded using a Horiba micro-Raman spectrometer, using a laser excitation source of 633 nm. The laser was focused onto the sample through an objective lens of 50X, providing a spot size of ~\u0026thinsp;1.54 \u0026micro;m. To assess signal intensity and consistency, the spectra were collected from multiple randomly selected sites across the drop-cast area of each substrate. To prevent the material from photothermal damage, the laser power was maintained at 5% during data acquisition. The acquired data were plotted using Origin software to get the final SERS spectra, and the enhancement factors were calculated.\u003c/p\u003e\n\u003ch3\u003eCharacterization:\u003c/h3\u003e\n\u003cp\u003ePrimarily, the optical properties of the laser-ablated nanosolids were investigated through UV-visible absorption spectroscopy by using a JASCO V-750 spectrometer. The UV spectrum of Au nanoparticles displays a pronounced absorption peak at ~\u0026thinsp;521 nm, confirming the presence of spherical nanoparticles. and for the Au-Ag solution, an absorption peak was observed at ~\u0026thinsp;470 nm. The morphological and dimensional features of the NPs were extensively examined via transmission electron microscopy. These analyses were performed with a JEOL JEM-F200 transmission electron microscope (TEM), operated by a high-voltage source. This TEM study allowed for the exploration of the shape and size distribution of NPs. The surface structure of the nanofiber sheet was studied using field emission scanning electron microscopy (FESEM). The FESEM imaging was carried out using a Carl ZEISS Ultra 55 microscope. Through this high-resolution imaging, the surface morphology of the nanofibers and the distribution of nanoparticles on the surface were investigated. The static contact angle measurements were carried out to verify the surface wettability and hydrophobicity (contact angle greater than 90\u0026ordm;). An ADCAM-02 apparatus was used to conduct this experiment, where a 2 \u0026micro;l distilled water droplet was dropped, and the contact angle measurements were carried out. The SERS studies of two dye molecules, NB and MG, were implemented to evaluate the enhancing capability of Au and Au-Ag nanoparticle-loaded nanofibers. SERS spectra were acquired with a Horiba micro-Raman instrument, using a monochromatic laser source of 633 nm. The laser was focused via a 50X objective lens, providing localised Surface plasmon excitation and efficient Raman signal collection from the analyte molecule present on the NPs-decorated fibre substrates.\u003c/p\u003e"},{"header":"Result and Discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eUV-Visible characterisation studies:\u003c/h2\u003e\u003cp\u003eThe UV-visible absorption data of fs ablated pure Au NPs and Au-Ag alloy NPs are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b), respectively. This absorption in metal NPs corresponds to surface plasmon resonance (SPR). SPR results from the collective oscillations of loosely bound surface electrons in metals when illuminated with appropriate photon sources. Similarly, in the case of semiconductors, UV absorption results from the valence band to the conduction band transition of electrons. In the case of pure Au NPs, the SPR absorption peak was found at ~\u0026thinsp;521 nm. Likewise, for Au\u003csub\u003e\u0026minus;\u003c/sub\u003eAg NPs, a single SPR peak was observed at ~\u0026thinsp;470 nm, signifying the formation of Au-Ag NPs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eTEM studies:\u003c/h3\u003e\n\u003cp\u003eThe structural features, such as shape, size, and interplanar spacing, were obtained from TEM imaging of NPs. The ablated colloidal NPs were drops cast onto the carbon-coated copper grids for TEM analysis and loaded into the apparatus. the loaded grids were exposed to a high-energy electron beam for data acquisition. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e illustrate the TEM images and detailed analysis of the ablated Au NPs and Au-Ag NPs, respectively. Figures\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a)-(c) and 5(a)-(c) illustrate the TEM images of Au NPs and Au-Ag NPs, respectively, at different magnifications. These TEM images were subsequently analysed through ImageJ software to evaluate particle size distribution, and the corresponding histogram plots are insets in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) and 5(a). Energy Dispersive X-ray (EDX) elemental mapping of Au nanocolloids and mass percentage are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d) and 4(e), respectively. Similarly, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(d) represents the EDX data of the Au-Ag nanocolloids, and Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(e) illustrates the elemental mass percentage of Au-Ag nanocolloids. EDX elemental mapping data showed that the Au nano colloids mainly consist of 93% of Au and only 6% of oxygen by mass. This low oxygen content suggests limited surface oxidation, pointing to its superior chemical stability in aqueous media. In comparison, the gold-silver alloy nanoparticles contained 28% gold, 22% silver, and a significantly higher oxygen content of 49%. The raised oxygen level in the alloy sample indicates greater surface oxidation, reflecting lower stability in aqueous media.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe average size measured corresponding to the Au NPs is ~\u0026thinsp;17 nm, and the estimated sizes are ~\u0026thinsp;17 nm and ~\u0026thinsp;22 nm for Au-Ag NPs, as illustrated in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) and 5(a) in histogram plots. HRTEM images of Au and Au-Ag NPs are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a) and Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a), respectively. The IFFT images of the HRTEM images were obtained from ImageJ processing, and are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b) and Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(c), depicting the d-spacing profile corresponding to it. Similarly, Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(b) and 7(c) display the IFFT images obtained from the HRTEM images and the d-spacing profile of Au-Ag NPs, respectively. The interplanar spacing estimated for Au NPs was 0.250 nm, and for Au-Ag NPs it was 0.252 nm. Bharathi \u003cem\u003eet al\u003c/em\u003e. reported an average size of ~\u0026thinsp;15 nm and an interplanar spacing of 0.230 nm for picosecond laser-ablated Au NPs [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Kuliliute \u003cem\u003eet al.\u003c/em\u003e recently synthesised Au NPs through femtosecond laser ablation in aqueous medium and found spherical particles, with a size distribution ranging from 4 nm to 50 nm [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Similarly, Hidayah et al. first ablated pure Au and Ag metal targets using a femtosecond laser in aqueous media, mixed the Au and Ag colloidal solution in a 1:1 ratio, and irradiated that mixture solution with a femtosecond laser beam. They reported an average diameter size of 15.03 nm Au-Ag (1:1) nanoparticles, synthesised through this irradiation [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eSEM \u0026 contact angle studies:\u003c/h3\u003e\n\u003cp\u003eThe FESEM images of plain PMMA fibers at different magnifications are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a) and 8(b). The inset histogram plot presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a) demonstrates the fibre diameter distribution of these PMMA fibers obtained using ImageJ processing of the FESEM Images. The average fibre diameter was found to be ~\u0026thinsp;642 nm. When compared to previous reports, Piperno et. al reported diameters ranging from 150 nm to 800 nm for electropolished PMMA fibers. Figures\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(c) and 8(d) illustrate the FESEM images of the Au NPs dropped on PMMA fibers. Simultaneously, Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(e) and 8(f) illustrate the FESEM images of Au-Ag alloy NPs dropped on PMMA nanofibers.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe higher magnification image in 8(d) illustrates a good quantity of adsorbed Au NPs on the PMMA fiber surface, which can aid in a superior SERS enhancement. Since this is a simple dropping technique (easier for the user to perform this step), the deposition of the NPs onto the nanofibers may not be extremely uniform. In the future, we will consider spin coating the NPs using different speeds. This surface adsorption of these drop-cast colloidal NPs showcases the hydrophobic behaviour of nanofibers. Further, the hydrophobic behaviour of PMMA nanofibers was verified through contact angle measurement with a 2 ml DW droplet. The acquired contact angle was ~\u0026thinsp;125\u0026ordm; at the moment of drop casting, and it decreased to ~\u0026thinsp;110\u0026ordm; after 30 minutes, as shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(a) and 9(c), respectively.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003eSERS studies:\u003c/h2\u003e\u003cp\u003eRigid SERS substrates have already proven their efficiency in lower concentration analyte detection and reproducibility [\u003cspan additionalcitationids=\"CR35 CR36 CR37\" citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. However, high manufacturing costs and difficulty in sample collection make these rigid substrates less preferable for real-time use. Considering this, cheaper, flexible SERS substrates have been extensively taken on for Sensing applications. Nanofibers are good options for flexible SERS substrates, facilitated by their high porosity, which enables a large surface area for nanoparticle loading and analyte adsorption. In our study, electrospinning fabricates flexible fibers used as substrates. These fibers were functionalized by drop casting plasmonic active Au NPs and Au-Ag NPs. These colloidal nanoparticles were deposited onto the hydrophobic fibers in varying volumes (1\u0026ndash;50 \u0026micro;l). The NPs were drop-cast on the nanofibers and placed on a hot plate for drying. This approach enabled a systematic study on the effect of nanoparticle loading on SERS performance. The Raman measurements were carried two NB (5 \u0026micro;M) and MG (5 \u0026micro;M), for each volume, using both PMMA-Au and PMMA-Au-Ag flexible substrates. A gradual increase in the SERS intensity was noticed with an increase in the NPs loading. Figures\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(a) and 10(b) showcase the related SERS plots using PMMA-Au substrates. Similarly, Figs.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(a) and 11(b) refer to PMMA-Au-Ag substrates. This occurrence of a saturated SERS signal can be attributed to the aggregation of nanoparticles, with an increase in the NPs volume. A saturation in the SERS intensities is observed beyond 20 \u0026micro;l in Figs.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(a) \u0026amp; 10(b). Similarly, Figs.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(a) and 11(b) show the SERS intensity saturation after 10 \u0026micro;l. Depending upon the volume of studies, the colloidal solution volume with better SERS intensity was finalized and used for concentration-dependent analysis. The concentration-dependent plots of the NB and MG with PMMA-Au substrates are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(c) and 10(d). Similarly, Figs.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(c) and 11(d) correspond to the concentration-dependent plots of NB and MG dyes with PMMA-Au-Ag substrates.\u003c/p\u003e\u003cp\u003eOur SERS studies revealed the observation of a superior enhancement with PMMA-Au-Ag substrates in the case of NB dye molecules. To examine the reproducibility of these flexible SERS substrates, the enhanced Raman signals were collected by focusing the laser source on different sites. The reproducibility of these flexible SERS substrates was estimated by evaluating the Relative Standard Deviation (RSD) among the SERS intensities corresponding to the most prominent peaks of the analyte. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(e) [NB (5 \u0026micro;M) data] and 10(f) [MG (5 \u0026micro;M) data] depict the RSD data collected using PMMA-Au substrate, and Figs.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(e) [NB (5 \u0026micro;M) data] and 11(f)[ NB (5 \u0026micro;M) data] present the RSD data collected from PMMA-Au-Ag substrates. PMMA-Au substrate uniformity in SERS signal was observed across the drop-cast region with reproducibility of 9.59% and 10.77% for NB (5 \u0026micro;M) and MG (5 \u0026micro;M), respectively. In parallel, PMMA-Ag-Au substates also exhibited comparable SERS intensity with reproducibility of 12.63% and 14.63% for analyte NB (5 \u0026micro;M) and MG (5 \u0026micro;M), respectively. This notable uniformity in the SERS signal is closely associated with the structural advantage of the electrospun PMMA nanofibers. The dense, porous, and interconnected framework of nanofibers promoted the even distribution of NPs during drop-casting. On the other hand, the hydrophobic behaviour restricted the spreading of aqueous colloidal NPs, allowing the confined accumulation of NPs in a particular region [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. This accumulation results in high NPs density, which leads to more SERS-active hotspots. The enhancement factors for both dyes are mentioned below in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e(a) and 12(b) depict the linear dependence of the SERS intensity versus concentration for the prominent peaks of MG (1616 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and NB (592 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), respectively, obtained using the PMMA-Au substates. Similarly, Figs.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e(c) and 12(d) depict the linear dependence of the peaks of MG (1616 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and NB (592 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), respectively, achieved using the PMMA-Au-Ag substates. It is evident that there is a good linear dependence of the SERS intensity on the concentration of the analytes used.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eThe enhancement factors of dyes obtained using Au and Au-Ag NPs.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSubstrate\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAnalyte\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEnhancement factor (Ef)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003ePMMA drop-casted with Au Nano colloid\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNile blue\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c3\"\u003e\u003cp\u003e2.27\u0026times;10\u003csup\u003e4\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMalachite green\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c3\"\u003e\u003cp\u003e1.03\u0026times;10\u003csup\u003e4\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003ePMMA drop-casted with Au-Ag Nano colloid\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNile blue\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c3\"\u003e\u003cp\u003e2.93\u0026times;10\u003csup\u003e5\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMalachite green\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c3\"\u003e\u003cp\u003e4.41\u0026times;10\u003csup\u003e4\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows that PMMA-Au-Ag substrates demonstrated superior enhancement to PMMA-Au substrates. This refinement can be attributed to the combined plasmonic effects of Au and Ag, which create a more intense electromagnetic hotspot on the surface. The strong plasmonic response of Ag enhances the field strength, while Au provides stability, resulting in stronger Raman signals and improved detection at lower concentrations.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe lowest detectable concentrations of the analyte were determined using the corresponding linear dependence of the SERS intensity versus concentration curves, applying the standard equation \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:LOD=3.3\\left(\\sigma\\:/S\\right)\\)\u003c/span\u003e\u003c/span\u003e. Here \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sigma\\:\\)\u003c/span\u003e\u003c/span\u003e represents the standard deviation of the blank measurements, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:S\\)\u003c/span\u003e\u003c/span\u003e denotes the slope of the linear dependence curve [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. The experimentally obtained LOD values, along with the corresponding theoretical predictions, are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e below. Our future studies will involve collecting the analytes by swabbing/swiping techniques and performing the SERS measurements on different hazardous molecules.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eSummary of the theoretical and experimental lowest detectable concentrations of analyte on NPs integrated PMMA nanofibers.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSubstrate\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAnalyte\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLowest concentration detected\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eLimit of detection (LOD)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003ePMMA drop-casted with Au Nanocolloids\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMG\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e200 nM\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e28 nM\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e50 nM\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e8 nM\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003ePMMA drop-casted with Au-Ag Nanocolloids\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMG\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e100 nM\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e20 nM\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e10 nM\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2 nM\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis work demonstrated the effective fabrication of Au and Au-Ag nanoparticles by focused femtosecond laser beam interaction with corresponding bulk material targets in an aqueous medium. The UV absorption spectra of these colloidal solutions depicted a peak absorbance at wavelengths\u0026thinsp;~\u0026thinsp;522 nm and ~\u0026thinsp;486 nm for Au and Au-Ag NPs, respectively. The single SPR peak in the Au-Ag colloidal solution confirms the presence of Au-Ag nanoparticles in solution. The TEM analysis revealed the average diameter distribution for Au NPs at ~\u0026thinsp;17 nm and ~\u0026thinsp;22 nm for Au-Ag NPs. The flexible PMMA nanofibrous sheet was prepared via the electrospinning technique and characterised through FESEM, before and after loading the NPs. The average fiber diameter was found to be ~\u0026thinsp;662 nm. Further, these PMMA fibers were loaded with Au and Au-Ag NPs and used for SERS analysis. This PMMA-Au fiber NPs composite successfully detected NB at 50 nM and MG at 200 nM as the lowest possible concentration. Similarly, for the PMMA-Au-Ag composite, the lowest detected concentration was 10 nM for NB and 100 nM for MG. Further optimization is conceivable with different sizes of these nanoparticles and the diameters of the nanofibers, resulting in a variety of loading scenarios (i.e., different numbers of hotspots). We believe that these low-cost, easy-to-prepare nanofibers will be attractive in practical sensing applications, especially based on the SERS technique, achieving ppm and/or ppb levels.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eV.R. Soma acknowledges the financial support from DRDO, India, through the DIA-CoE, ACRHEM. V.R. Soma also thanks the University of Hyderabad, India, for the IoE (Institute of Eminence) project [# \u003cb\u003eUOH/IOE/RC1/RC1-20-016\u003c/b\u003e]. The IoE project was granted by the Ministry of Education, Government of India, vide MHRD, India notification F11/9/2019-U3(A). V.R. Soma also acknowledges the IOE, UoH, for funding through a collaborative Inter-Institutional Research Clusters Project \u003cb\u003eUoH-IOE-IIRC-24-007\u003c/b\u003e.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eNP did the measurements, analyses, and wrote the manuscript.VRS supervised, procured funding, gave the ideas, discussed the results and wrote the manuscript\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eWe acknowledge the use of ChatGPT to correct the manuscript for any grammatical errors and paraphrasing (the introduction part). V.R. Soma thanks Dr. Bikash Ghose, HEMRL, Pune, India, and Director, DIA-CoE (Dr. S.C. Bhattacharya), University of Hyderabad, India, for fruitful discussions, suggestions, and encouragement.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSharma B, Frontiera RR, Henry AI, Ringe E, Van Duyne RP (2012) SERS: Materials, applications, and the future, Mater. Today 15 16\u0026ndash;25. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/S1369-7021(12)70017-2\u003c/span\u003e\u003cspan address=\"10.1016/S1369-7021(12)70017-2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSchatz GC, Young MA, Van Duyne RP (2006) Electromagnetic mechanism of SERS. 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Chronicles Young Sci 2:21. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.4103/2229-5186.79345\u003c/span\u003e\u003cspan address=\"10.4103/2229-5186.79345\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"plasmonics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"plas","sideBox":"Learn more about [Plasmonics](https://www.springer.com/journal/11468)","snPcode":"11468","submissionUrl":"https://submission.nature.com/new-submission/11468/3","title":"Plasmonics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"femtosecond laser ablation, electrospinning, surface-enhanced Raman spectroscopy, PMMA-nanofibers","lastPublishedDoi":"10.21203/rs.3.rs-7436815/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7436815/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFlexile SERS substrates accompany distinct advantages for real-time detection, not only because of their high porosity, but also their ability to adapt to irregular surfaces and withstand bending or movement, making them ideal for practical, on-the-go sensing applications. The present work includes the fabrication of flexible substrates by combining electro-spun PMMA polymer nanofibers with femtosecond pulse laser ablated pure Gold (Au) and bimetallic (Au-Ag) NPs. Hydrophobic PMMA fibers were functionalised by drop casting plasmonic-active Au and Au-Ag NPs for surface-enhanced Raman scattering/spectroscopy (SERS) studies. A micro-Raman spectrometer (M/s Horiba) was used for the SERS data acquisition for two dye molecules, Nile blue (NB) and malachite green (MG). Au-loaded PMMA fibers detected up to 50 nM concentration of Nile blue and 200 nM concentration of MG, and also demonstrated a good reproducibility with R\u003csup\u003e2\u003c/sup\u003e values of 9.59% and 10.77% for NB (5 \u0026micro;M) and MG (5 \u0026micro;M), respectively. Similarly, PMMA-Au-Ag substrates detected NB with 10 nM concentration and Mg with 100 nM concentration as the lowest detection and showed reproducibility of 12.63% and 14.63% for NB (5 \u0026micro;M) and MG (5 \u0026micro;M), respectively.\u003c/p\u003e","manuscriptTitle":"Gold, Gold-silver Alloy Nanoparticle-functionalized PMMA Nanofibers for Ultrasensitive SERS-based Detection of Nile Blue and Malachite Green","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-12 15:20:19","doi":"10.21203/rs.3.rs-7436815/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-16T18:26:26+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-16T18:19:56+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-15T16:40:47+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"158580546721348314997098902411820532950","date":"2025-09-13T06:11:18+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-12T15:09:01+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"63880483906364593357798847685955390281","date":"2025-09-10T03:44:21+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-08T03:00:35+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"322984800056967770990942962613124354097","date":"2025-09-08T02:19:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"111299723760613790358817993949574407179","date":"2025-09-07T23:09:26+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"284718128561890421409145338394968895219","date":"2025-09-07T18:34:09+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"210629712239901715540906771559139916612","date":"2025-09-07T15:24:29+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-07T15:20:24+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-26T02:35:28+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-26T02:35:04+00:00","index":"","fulltext":""},{"type":"submitted","content":"Plasmonics","date":"2025-08-22T18:13:42+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"plasmonics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"plas","sideBox":"Learn more about [Plasmonics](https://www.springer.com/journal/11468)","snPcode":"11468","submissionUrl":"https://submission.nature.com/new-submission/11468/3","title":"Plasmonics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"5948827c-0091-4cfa-971f-a7a8b188a9ac","owner":[],"postedDate":"September 12th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-10-20T16:00:57+00:00","versionOfRecord":{"articleIdentity":"rs-7436815","link":"https://doi.org/10.1007/s11468-025-03298-5","journal":{"identity":"plasmonics","isVorOnly":false,"title":"Plasmonics"},"publishedOn":"2025-10-14 15:57:27","publishedOnDateReadable":"October 14th, 2025"},"versionCreatedAt":"2025-09-12 15:20:19","video":"","vorDoi":"10.1007/s11468-025-03298-5","vorDoiUrl":"https://doi.org/10.1007/s11468-025-03298-5","workflowStages":[]},"version":"v1","identity":"rs-7436815","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7436815","identity":"rs-7436815","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}