Electrostatic Assemblies of Well-Dispersed AgNPs on the Surface of

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Electrostatic Assemblies of Well-dispersed AgNPs on the Surface of Electrospun Nanofibers as Highly Active SERS Substrates for Wide Range pH Sensing Tong Yang, Jun Ma, Shu Jun Zhen, and Cheng Zhi Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03720 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 25, 2016

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ACS Applied Materials & Interfaces

Electrostatic Assemblies of Well-dispersed AgNPs on the Surface of Electrospun Nanofibers as Highly Active SERS Substrates for Wide Range pH Sensing Tong Yang,† Jun Ma,‡ Shu Jun Zhen,*,‡ and Cheng Zhi Huang*,†,‡ †

Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest

University), Ministry of Education, College of Pharmaceutical Science, Southwest University, Chongqing 400715, P. R. China. ‡

Chongqing Key Laboratory of Biomedical Analysis (Southwest University), Chongqing

Science & Technology Commission, School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400716, P. R. China Corresponding Author * E-mail: [email protected], Tel: (+86) 23 68254659, Fax: (+86) 23 68367257.

ACS Paragon Plus Environment

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ABSTRACT: Surface enhanced Raman scattering (SERS) has shown high promise in analysis and bioanalysis, wherein noble metal nanoparticles (NMNPs) such as silver nanoparticles were employed as substrates because of their strong localized surface plasmon resonance (LSPR) properties. However, SERS-based pH sensing was restricted because of the aggregation of NMNPs in acid medium or biosamples with high ionic strength. Herein, by using the electrostatic interaction as a driving force, AgNPs are assembled on the surface of ethylene imine polymer (PEI)/polyvinyl alcohol (PVA) electrospun nanofibers, which are then applied as high sensitive and reproducible SERS substrate with an enhancement factor (EF) of 107 ~ 108. When p−aminothiophenol (p−ATP) as an indicator with its b2 mode, a good and wide linear response ranging from 1.98 to 11.20 of pH could be available, and the as-prepared nanocomposite fibers then could be fabricated an excellent pH sensors in complicate biological samples such as urine considering that the pH of urine could reflect the acid−base status of persons. This work not only emerges a cost-effective, direct and convenient approach to homogeneously decorate AgNPs on the surface of polymer nanofibers, but also supplies a route for preparing other noble metal nanofibrous sensing membranes. KEYWORDS: electrostatic assemblies, silver nanoparticles (AgNPs), electrospun nanofibers, SERS substrates, pH sensing


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INTRODUCTION Surface enhanced Raman scattering (SERS) is a highly sensitive and noninvasive vibrational spectroscopic technique, which can rapidly provide intrinsic fingerprint information of analytes or bioanalytes and also be used for varieties of molecular/cellular sensing or imaging.1-4 One classic feature of the highly active SERS substrates is the presence and spatial distribution of numerous “hot junctions” between the closely packed metallic nanocrystals, resulting from the contribution of localized surface plasmon resonance (LSPR).5 In general, noble metal nanoparticles (NMNPs) such as silver and gold nanoparticles (AgNPs, AuNPs) have been used as conventional SERS substrates. The aggregates of NMNPs are desirable in SERS, however, the bad dispersity of NMNPs substrates in complicate samples lead to poor reproducibility and limit their wide applicability. In order to overcome this hurdles, solid-state SERS active substrates with the high sensitivity and reproducible Raman enhancing ability have been developed by assembling or growing NMNPs on some solid supports including anodic aluminum oxide (AAO) nanochannels,6 common rose petals,7 polymeric thin films,8 AgNPs/polyacrylonitrile nanohump array films,9 zinc oxide (ZnO)-mesoporous nanosheets grafted on flexible carbon fiber cloth,10 and slippery liquid-infused porous SERS.11 These solid-state SERS substrates exhibit a sensitivity to target molecules and the more uniformity of AgNPs on supports will lead to reproducible and stable SERS signals. Electrospun fibers are interesting solid supports. Electrospinning has been considered as one of the most direct and versatile route to fabricate nonwoven, large surface area to volume ratio, high porosity and three dimensional (3D) networked nanofibrous meshes.12-14 Incorporating functional nanocrystals to electrospun nanofibers opens a broad avenue of applications such as fingerprint recognition,15 bio- and chemical sensor,16, immunoassay,19 catalysis,20 and biomedical fields21,

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nanofibrous filter membrane,18

. Previously, we have successfully

prepared two AgNPs-doped electrospun nanofibrous SERS substrates.23, 24 The two hydrophilic molecules―polymethacrylic acid and agar were doped into polymeric nanofibrous hosts to greatly improve the density of AgNPs and proved to be a benefit to the SERS measurements.


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However, this strategy requires the analytes to sufficiently permeate into the nanofibers and to be adjacent to the AgNPs. The results in lowering the overall availability of AgNPs encapsulated in the nanofibers, which aren’t suitable for more target molecules. In order to overcome these obstacles, efforts such as chemical reduction,25 layer-by-layer assembly,26 electrodeposition,27 were taken to immobilize NMNPs onto the surfaces of the electrospun nanofibers. Among the different existing methods, electrostatic adsorption between NMNPs and electrospun nanofibers is the most direct, versatile and nonspecific interaction, which can keep the uniformity of NMNPs and improve the Raman enhanced effect.

Scheme 1. Schematic Illustration of the Preparation of AgNPs/PEI/PVA Nanofibrous SERS Substrates and Its Application as pH sensor With this purpose, we herein present a new route to the fabrication of the versatile, highly sensitive and well reproducible SERS substrates, which are composed of the positively charged PVA nanofibers functionalized by PEI and the negatively charged AgNPs assembled by the driving force of electrostatic interactions (Scheme 1). The results show that an appropriate density and well-distribution of AgNPs on the surface of PEI/PVA nanofibers guarantees the high sensitivity and reproducible SERS substrate with an enhancement factor (EF) of 107 ~ 108. We have then demonstrated the applicability of the as-prepared AgNPs/PEI/PVA nanofibrous membranes as a SERS-based pH sensor with the use of p−aminothiophenol (p−ATP) as indicator. We have successfully obtained a linear response in the wide range of pH values


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comprised between 2.56 and 11.20. Furthermore, the as-prepared nanocomposite fibers was proved as pH sensors in real biological environment such as human urine providing relevant information that can reflect the acid-base status, renal disease or urinary tract infection of a person.

EXPERIMENTAL SECTION Materials. Poly(vinyl alcohol) powder (PVA, 99% hydrolyzed, average MW = 89, 000 ~ 98, 000) was purchased from Sigma-Aldrich. Silver nitrate (AgNO3 AR, 99.8%) and trisodium citrate (Na3C6H5O7·2H2O AR) were obtained from Shanghai Shenbo Chemical Engineering Co. Ltd. (Shanghai, China) and Tianjin Regent Chemical Reagent Co., Ltd. (Tianjin, China), respectively. Absolute ethanol (C2H5OH, 99.7%) and glycerol (C3H8O3) were acquired from Amresco Company. Glutaric dialdehyde (GA, 25%) was chosen as a chemical cross-linking solvent and was supplied from Tianjing Guangfu Technology Development Co. Ltd.. Ethylene imine polymer (PEI, average Mw = 70, 000), rhodamine 6G (R6G) and p−aminothiophenol (p−ATP) were purchased from Aladdin Chemistry Co., Ltd (Shanghai, China). All chemicals were used as received without further treated. Milli-Q purified water (18.2 MΩ·cm−1) was used throughout the experiment. Apparatus. The UV-vis absorption spectra of AgNPs were measured with UV−3600 spectrophotometer (Hitachi Ltd., Tokyo, Japan). The morphologies of the as-fabricated nanofibers and nanoparticles were imaged by using a scanning electron microscopy (SEM, S4800, Hitachi, Japan). The elemental composition of samples was analysed using an X-ray energy dispersive spectroscopic (EDS) detector attached to the SEM. The samples were attached to the surface of monocrystalline silicon wafer. The situ morphology and crystallographic properties of crystal samples were determined by transmission electron microscopy and high resolution transmission electron microscopy (TEM and HRTEM, G2 F20 S-TWIN, Tokyo, Japan), with a suitable selected area electron diffraction (SAED). X-ray powder diffraction (XRD) patterns were obtained by a Shimadzu XRD-7000 (Beijing Purkinje General Instrument Co. Ltd., China) with Cu-Kα (1.5405 Å) radiation source under the operating voltage and current of 40 kV and 50 mA. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCRLRB250X (America, USA) spectrometer with a standard Al K source (hν = 1486.6 eV). Functional groups analysis of relevant composites were recorded by a Fourier transform infrared


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spectrometer (Shimadzu, 8400S, Japan) from 4000 to 600 cm-1 at room temperature. All Raman spectra were collected on a LabRAM−HR Raman spectrometer (HORIBA Jobin Yvon, France) with an excitation source of 532 nm, accumulation time: 4 s. Synthesis of Negatively Charged AgNPs. . The monodispersed quasi-spherical and sizecontrollable AgNPs were synthesized by using the well-established two-step seed-mediated growth protocol.28 In a typical procedure, a 50 mL glycerol−water mixture (20 vol % glycerol) was vigorously and continuously stirred in a 100 mL conical flask and heated up to 100 oC. When the temperature was steady, 90 µL of AgNO3 (0.1g/mL) was added into the hot blended solution. Then, 1 mL of trisodium citrate (3 wt %) were added to the solvent. After 1 h, 20 µL of AgNO3 (0.1 g/mL) and 1 mL trisodium citrate (3 wt %) were added into the blended solvent, sequentially. The latter step was repeated three times every five minutes. The products were cooled down to ambient temperature and stored at 4 oC before use. Finally, the molar concentration of AgNPs were determined by the Lambert-Beer’s Law on the basis of relevant works.29, 30 Electrospinning. Typically, PVA powders were dissolved in deionized water (DIW) for 3 h at 90 oC with concentrations of 12 wt %. The PEI (50 wt %) was mixed with the former solution (the PEI/PVA mass ratio of 1:3) under vigorous stirring at room temperature for about 8 h. The ζ-potential measurement of the obtained PEI/PVA electrospun solution was performed using a Zeta-sizer Nano-ZS90 instrument (Malvern Inc., UK). Then the homogeneous and viscous electrospun solution was filled into a 10 mL plastic syringe with the 0.8 mm diameter of bluntended needle. All nanofibers were electrospun prepared using a commercial electrospinning equipment (DNF−001, Beijing Kaiweixin Technology Co. Ltd., China) under the high-voltage of 25.0 kV. The needle was located at a distance of 20 cm from the grounded collector wrapped with an aluminium (Al) foil. The rate of the syringe pump was set to 1.8 mm/h. At last, the PEI/PVA nanofibers were cross-linked using GA vapor in a vacuum desiccator for 10 h at 60 oC. Then, the nanofibrous membranes were washed with distilled water three times to remove redundant GA. The cross-linked PEI/PVA nanofibrous mats were dried under vacuum and stored in a desiccator before use. Casting PEI/PVA Films. For the preparation of positively charged PEI/PVA films, 400 µL of the PEI/PVA viscous solution was dropped on a 2 cm × 2 cm clean slide glass and spin coated by spin coater (KW-4A, Kunshan Lidian Precision Instrument Co. Ltd., Jiangsu, China) at 1000


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r/min for 10 s. The products were left to dry at room temperature form the thin PEI/PVA film, and were further cross-linked under GA vapor as described above. Immobilization of AgNPs on PEI/PVA Nanofibers. The cross-linked nanofibrous mats were cut into small pieces with an approximate 4 cm2 (length 4 cm, width 1 cm). The fibrous membrane was subsequently immersed into the desired volume and concentration of negatively charged silver colloid (AgNPs concentration, 0.85 nM) under vigorous shaking in an incubator shaker for 8 h at 30 oC until the nanofibrous mats changed from pale yellow to dull grey. The nanofibrous membrane was then rinsed with distilled water to remove the loosely bound AgNPs. Lastly, the membranes were allowed to dry at room temperature and stored under ambient condition. SERS Performance. The different concentration of R6G and p−ATP solution were used as the reference signal of molecule to assign the SERS activity of AgNPs/PEI/PVA nanofibrous substrates. At first, the SERS activities of the nanofibrous substrates with different concentrations of AgNPs were investigated by using p−ATP (10−5 M) as the reporter molecule. Then, 50 µL of R6G (10−8 M) and R6G (0.1 M) aqueous solutions was prepared by drop-casting a 0.25 cm2 surface of AgNPs/PEI/PVA and PEI/PVA hybrid nanofibrous mat, respectively. After being dried in the air, SERS spectrums of R6G (10−8 M) on fibrous mat and R6G (0.1 M) on the films were acquired at randomly selected spots. The SERS enhancement factors (EF) were then calculated. Laser wavelength, 532 nm; power, 28 mW; lens, 50× objective; acquisition time: 4 s. All SERS spectral were collected after baseline correction using a 5th order polynomial leastsquares fitting algorithm. pH Sensing Based on the SERS Signal of p−ATP on Membranes. Britton−Robison (BR), phosphate buffer saline (PBS), citric acid−phosphate buffer solutions with well-defined pH ranging from acidity to basicity were prepared by mixing their relevant solution with different volume ratios. The pH values of various buffers were measured with a pH 510 precision pH meter (Eutech, USA). Mixture solutions containing different pH buffer solutions and p−ATP solution (10−4 M) were incubated for 2 h (the final concentration of p−ATP: 10−5 M, the volume ratio of buffer solution/p−ATP: 9:1). Then, the AgNPs/PEI/PVA nanofibrous mats were immersed in the above solutions for 3 h, respectively. The mats were finally removed from their respective solution and dried in the air. SERS spectra of p−ATP at different pH were then measured.


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The developed substrates for pH sensor based on SERS signals were tested to measure the pH value of six urine samples collected from normal volunteers. All experiments were performed in triplicate or more.

RESULTS AND DISCUSSIONS Immobilization of Negatively Charged AgNPs onto the Surface of Positively Charged PEI/PVA Electrospun Nanofibers The monodispersed quasi-spherical AgNPs were successfully synthesized by using the wellestablished two-step seed-mediated growth protocol (Figure S1, the detailed discussion in Supporting Information).28 Under specific experimental conditions, smooth and uniform PEI/PVA nanofibers with a mean diameter of 391 ± 23 nm and high porosity were fabricated (Figure 1A). However, PVA and PEI are hygroscopic high molecular polymers and it is necessary to avoid the morphology deformation of the prepared PEI/PVA nanofibers due to atmospheric moisture. The conglutination of bare PEI/PVA nanofibers between adjacent nanofibers and the formation of non-uniform diameters were presented when subjected to the atmospheric moisture for a period of time (Figure S2A, Supporting Information). A good moisture resistant nanofibers could be obtained glutaraldehyde (GA) vapor.31 After crosslinking with GA, the PEI/PVA nanofibers maintain their uniform and smooth surface with the diameter of 398 ± 27 nm (Figure 1B and Figure S2B, Supporting Information). The chemical functional groups were characterized by FTIR (Figure 2C). The characteristic peak at 1617 cm−1 assigned to the −C=N stretching vibration (aldimine linkage), which revealed the successful crosslinking reaction between the amines in PEI and the aldehyde in GA.32 Another weaker absorption peak at 1050 cm−1 confirms the formation of the ether linkage (C−O−C), which stems from the reaction between the hydroxyl groups in PVA and the aldehyde groups in GA.33 The color variation (Figure S3, in Supporting Information) of the PEI/PVA nanofibers after crosslinking by GA vapor are attributed to the formation of aldimine linkage, consisting of the main component of the water-stable PEI/PVA nanofibers. In addition, the doped PEI was not only used as a simple crosslinking agent, but also as an excellent surface modifying agent. Because of the high cationic charge density along the PEI chains, branched polymer―PEI have been directly employed to modify the surface charges of nanomaterials, nanofibers, carbon materials among other examples.34-36 Similarly, the


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introduction of PEI in PVA as host polymers has induced a high cationic charge density on the surface of the PVA nanofibers. Based on the average potential of PEI/PVA electrospun solution (+34.4 mV) and silver colloidal solution (−35.1 mV) (Figure 1D, Figure S4, Supporting Information), the negatively charged AgNPs could directly and tightly immobilize onto the surface of PEI/PVA nanofibers through a strong electrostatic interaction.

Figure 1. Characterization of water-stable PEI/PVA nanofibers. SEM images and morphology of PEI/PVA nanofibers: (A) before cross-linking by GA vapor, (B) after cross-linking by GA vapor, the top right insets are the their diameter distribution histogram of nanofibers. (C) FTIR spectra of (a) PEI/PVA nanofibers and (b) cross-linked PEI/PVA nanofibers. (D) ζ-potential measurements of PEI/PVA electrospun solution (dark cyan histogram) and silver colloidal solution (pink histogram). Visually, the typical pale yellow electropun nanofibrous membranes turned into grey after decorating with AgNPs, indicating that AgNPs could assemble on the surface of PEI/PVA nanofibers (Figure S3B and C, Supporting Information). Figure 2A and B show that the nanofibers still maintain a smooth and uniform morphology after immersing in silver colloidal aqueous solution not evidencing any conglutination between adjacent nanofibers. In addition, the AgNPs are homogeneously distributed on the surface of nanofibers with a high density. We investigated the influence of the concentrations of silver colloids on the assembling distribution of AgNPs onto the surface of PEI/PVA nanofibers (Figure S5, Supporting Information). On the


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one hand, a growing number of loaded AgNPs appeared on the nanofibers while increasing the concentration of AgNPs. On the other hand, the exterior of PEI/PVA electrospun nanofibers were gradually transforming from a smooth into a coarse appearance. Figure 2C displays the further magnified HRTEM image of the AgNPs/PEI/PVA nanofibers. The lattice fringe with an interplanar spacing of 0.23 nm were measured, which is consistent with the (111) lattice plane of Ag0.37 The SAED picture of the AgNPs from nanocomposite nanofibers confirmed that the AgNPs had single crystal nature with face centered cubic (fcc) structure on the appearance of nanofibers (Figure 2D).38 Consequently, from the XRD pattern of the nanocomposite fibrous mesh (Figure S6A, Supporting Information), the typical four diffraction peaks with 2θ values centered at around 38.2o, 44.3o, 64.5o and 77.3o correspond respectively to the (111), (200), (220) and (311) crystal planes of the characteristic fcc structure of Ag0. The splitting of the Ag 3d orbital region doublet from the high-resolution XPS spectra is 6.0 eV based on the binding energies difference between Ag 3d5/2 (368.1 eV) and Ag 3d3/2 (374.1 eV) (Figure S6B, Supporting Information). The EDS analysis also proves the existence of silver elements (Figure S7). All of the above analysis confirm that the structure, property and morphology of AgNPs are mentioned even after decoration of the nanofibers.

Figure 2. Characterization on AgNPs/PEI/PVA composite nanofibers. (A) SEM and (B) TEM images of AgNPs/PEI/PVA nanofibers, (C) HRTEM and (D) SAED image of AgNPs on the surface of PEI/PVA nanofibers. The insets are the magnified SEM and TEM images of


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AgNPs/PEI/PVA nanofibers. The AgNPs/PEI/PVA nanofibers were prepared by decorating 0.6 nM AgNPs on PEI/PVA nanofibers. SERS Performance of AgNPs/PEI/PVA Nanofibrous Membrane as an Active Substrate Considering that the content of AgNPs on the nanofibers will have an impact on SERS performance, the composite nanofibers with different contents of AgNPs as SERS active substrates were investigated by using p−ATP as a model Raman reporter molecule. Figure 3A displays the SERS spectra for 1 × 10−5 M p−ATP with eight different AgNPs/PEI/PVA nanofibers, which stemmed from immersing PEI/PVA nanofibers into different silver colloids (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 nM). The characteristic bands of p−ATP in a SERS spectrum at 1578 (νC‒C), 1191 (νC‒S) and 1075 cm−1 (γC‒S) assigned to the a1 vibrational modes, and 1430 (νN‒N + βC‒H), 1392 (νC‒C + βC‒H), 1143 cm−1 (νC‒N + βC‒H) assigned to the b2 vibrational modes (ν = stretching vibration, β = bending vibration).24, 39-41 The optimal concentration of the AgNPs was 0.6 nM and was obtained by calculating and analyzing statistically the intensity of the major peaks of the p−ATP probes molecules on the above nanofibers (Figure 3B). At the same experimental parameters and spectral pattern, the most remarkable SERS signals for p−ATP signal are from the crosslinked PEI/PVA nanofibrous mats with 0.6 nM Ag colloid. Furthermore, the different batches of nanofibers can also give the relatively stable SERS signals for the characteristic bands of p-ATP (1.0×10-5 M) (Figure S8, in Supporting Information).

Figure 3. SERS activity of AgNPs decorated on the surface of PEI/PVA nanofibers. (A) SERS spectra of p−ATP (10−5 M) on the surface of different electrospun nanofibers, which are in accordance with Figure S5, respectively. (B) Statistic histograms of SERS intensity of characteristic peaks about p−ATP on the different AgNPs/PEI/PVA nanofibrous mats. Error bars represent the standard deviation of six measurements at different selected spots on


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AgNPs/PEI/PVA nanofibrous mats. Laser wavelength: 532 nm; Power: 28 mW; Lens: 50× objective; Acquisition time: 4 s. Figure 4 shows the SERS spectra were collected on different parts cut from the same electrospun fibrous membranes with varying concentrations of p−ATP (A) and R6G (B) (cp−ATP: from 10−6 to 10−8 M, cR6G: from 10−8 to 10−11 M). The SERS intensity of the spectrum obtained for the sample prepared with 10−8 M of p−ATP is very weak, reaching the lowest detection limit of p−ATP. The prominent bands of R6G around at 617, 778, 1133, 1185, 1317, 1366, 1514, 1578 and 1657 cm−1 are clearly distinguished in Figure 4B. The R6G, as a cationic fluorescent dye molecule, allows a good signal-to-noise ratio with a concentration as low as 10−10 M on the as-prepared nanofibrous mats. It is important notice that high sensitivity of the AgNPs/PEI/PVA nanofibrous SERS substrate were confirmed from the two experiments.

Figure 4. SERS activity of optimal AgNPs/PEI/PVA nanofibers. SERS spectra of (A) p−ATP (10−5 M, 10−6 M, 10−7 M) and (B) R6G (10−7 M, 10−8 M, 10−9 M, 10−10 M, 10−11 M). Preferable reproducibility was demonstrated by the overlapping SERS spectra of (C) p−ATP (1 × 10−5 M) and (D) R6G (1 × 10−8 M). Both were recorded on 20 randomly selected spots on the AgNPs/PEI/PVA nanofibrous mats (0.5 cm × 0.5 cm, the area: 0.25 cm2), respectively. (E) SERS spectra of R6G (10−8 M) on AgNPs/PEI/PVA fibrous mats (violet curve) and 0.1 M R6G on PEI/PVA fibrous mats (pink cure). (F) Contrastive experiments: SERS spectra of R6G (10−8 M) on the different structure of SERS substrates, including AgNPs/PEI/PVA electrospun


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nanofibrous substrates (violet cure), AgNPs/PEI/PVA planar film (pink curve), solid AgNPs on silicon wafer (dark cyan curve), silver colloidal solution (pale blue curve). Laser wavelength: 532 nm; Power: 28 mW; Lens: 50× objective; Acquisition time: 4 s. The excellent reproducibility of the Raman signals of the analytes on the as-prepared SERS substrates is an obligatory prerequisite before employing SERS as a routine analytical tool, and is evaluated by the relative standard deviation (RSD) of the characteristic SERS peaks of the analytes.24, 42 SERS spectra of p−ATP (1 × 10−5 M) and R6G (1 × 10−8 M) molecules from 15 and 20 randomly selected spots on the membranes were recorded under identical experimental conditions, respectively (Figure 4 C and D). The results of the two Raman probes revealed the satisfactory reproducibility across the whole area of the optimized AgNPs/PEI/PVA nanofibrous substrate with a RSD < 14.1% (Table S1, Supporting Information), which is far below the 22% obtained in our previous work.24 We have also quantitatively estimated the enhancement factor (EF) values of the optimized AgNPs/PEI/PVA nanofibrous mat as SERS-active substrate by analyzing the four major peaks of R6G at 617, 778, 1366, 1657 cm−1. The 1 × 10−8 M and 0.1 M R6G are served as SERS and normal Raman reporter molecules, respectively. In Figure 4E, the EF values of the AgNPs/PEI/PVA nanofibrous substrate were estimated to be 107 ~ 108 depending on the choice of the characteristic Raman peaks and relevant literature (Table S2, the detailed calculation can be consulted in the Supporting Information).43 All SERS spectral were collected after baseline correction to enhance the signal-to-noise ratio (Figure S9 in Supporting Information). From these outcomes, we concluded that (1) the assembling method of AgNPs on the surface of PEI/PVA nanofibers via electrostatic assembly greatly improves the sensitivity and enhances the efficiency of the electrospun nanofibrous substrate for SERS measurements; (2) the lower RSD value reflects the excellent reproducibility of the as-fabricated AgNPs/PEI/PVA substrate, and also reports the overall homogeneity and reliability of the AgNPs assembly on PEI/PVA nanofibrous mats as SERS-active substrates. In order to evaluate the superior SERS performance of the electrospun nanfibrous substrates, in this work, we prepared three other different SERS substrates which are AgNPs/PEI/PVA casting films, a solid AgNPs and a plain silver colloidal solution for comparison experiments. As can be seen from Figure 4F, the SERS signal of R6G (1 × 10−8 M) recorded on the electrospun nanofibrous substrate exhibits a predominant enhancement in intensity relative to that of the


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planar film, solid AgNPs substrate deposited on silicon pellet and direct AgNPs colloidal substrate, which are ascribed to the specific structures of the electrospun nanofibers with large surface area, high porosity and 3D network nanostructure. pH Sensing Based on the Sensitive SERS Signal of p−ATP on AgNPs/PEI/PVA Nanofibrous Mats and its Mechanism As for the SERS-based pH sensing performance of p−ATP, it is necessary to investigate and discuss its sensing mechanism. In 1993, Hill et al. have pointed out that the p−ATP exists under two molecular structures: the aromatic state in acidic solutions and the quinonoidic state in neutral or alkaline solutions.44 They suggested that the former state in acidic solution was attributable to the ammonium form, which results from the protonation of the quinoid form in p−ATP. Nowadays, it is generally accepted that the two vibrational modes including a1 and b2 types in SERS spectra of p−ATP stem from electromagnetic and chemical enhancement mechanism.45-47 Extensive experimental evidences and theoretical studies showed that the b2 modes are caused by the formation of p, p’‒dimercaptoazobenzen (DMAB), which is produced by a photo-reduced selective catalytic coupling reaction of p−ATP during the SERS measurements.48, 49 Furthermore, the b2 mode bands for p−ATP are affected by the pH because of the protonation and deprotonation of the amine group.50

Figure 5. Schematic illustration of the molecular structure of p−ATP-AgNPs/PEI/PVA nanofibers in acidic solution (A) and the formation of p, p’‒dimercaptoazobenzen (DMAB) in alkaline solution (B). In view of the points mentioned above, the SERS-based pH sensing performance of p−ATP on the AgNPs/PEI/PVA nanofibrous mats can be depicted as follows. At first, the relevant states of p−ATP were formed by pH variation of the mediums. Secondly, the AgNPs/PEI/PVA nanofibrous mats were soaked into the above solutions; the p−ATP molecules could tightly


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adsorb onto the surface of the composite nanofibers by a strong affinity between the thiol group and the AgNPs. In other words, the amine group in p−ATP tends to be protonated with the increasing pH, suggesting that more p−ATP on AgNPs/PEI/PVA nanofibrous substrate will turn into DMAB by photo-reduced selective catalytic coupling reaction during SERS measurements (as shown in Figure 5). In our SERS experiments, the laser power is kept the same for all samples.

Figure 6. The resultant calibration curves and interference tests. (A) SERS spectra of p-ATP (10−5 M) recorded on the AgNPs/PEI/PVA nanofibrous membranes in various pH values: (a) 1.81, (b) 2.09, (c) 2.56, (d) 3.29, (e) 4.10, (f) 5.02, (g) 6.09, (h) 7.00, (i) 7.96, (j) 8.95, (k) 9.62, (l) 10.38, (m) 11.20, (n) 11.98. (B) pH calibration curve obtained by plotting the ratio of peaks at 1430 and 1075 cm−1 against the pH values of the BR buffer solution. Relative SERS intensity (I1430/I1075) response of p−ATP (1.0 × 10-5 M) with different coexistences (1.25 × 10-4 M, K+, Na+, Ag+, Ca2+, Mg2+, Zn2+, Cu2+, Cd2+, Ni2+, Mn2+, Pb2+, Cr3+, Fe3+ and Al3+) at the different BR pH values: (C) pH 5.42; (D) pH 8.97. The inset is the pH sensing calibration ranging from 1.81 to 11.98. Laser wavelength: 532 nm; Power: 28 mW; Lens: 50× objective; Acquisition time: 4 s. The pH-dependence of the “b2 type” bands of the p−ATP molecule has been reported previously, and the SERS intensity of “b2 modes” bands increase together with the value of pH under the same laser power.47,

51

p−ATP molecules in BR solutions at different pH values

ranging from 1.81 to 11.98 were captured on the AgNPs/PEI/PVA nanofibrous substrates (Figure


15


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6). One can easily notice that the SERS intensities of the “b2 type” bands (1430, 1143 cm−1) gradually increase with the growing pH values indicating that the “b2 type” are sensitive to pH values. According to previous reports, the SERS band at 1075 cm−1 (γC‒S) can be generated by both p−ATP and DMAB with a comparable intensity and are independent from the variation of pH, so this band can be used as a control to compare the intensity changes of b2 mode bands (1430 cm−1).47 We calculated the intensity ratio (I1430/I1075) of the selected pair of bands at 1430 and 1075 cm−1 at the different pH values to monitor the SERS-based pH sensing by using p−ATP. The intensity ratio of I1430/I1075 is plotted against the pH values in Figure 6B. As expected, the correlation is very nice (Y = − 0.74 + 0.32 pH, R2 = 0.9944, Y stands for the intensity ratio of I1430/I1075) on a wide pH range (2.56 to 11.20). Analogously, the pH sensor was also sensitive to the other commonly used buffer solutions, indicating that the sensing action are owing to the varied pH values (Figure S10, Supporting Information). In addition, to assess the influence of the concomitant metallic cations for p−ATP, the SERS measurements of p−ATP with the coexistence of the fifteen metallic cations in acidic and alkaline solutions were conducted, respectively. In marked contrast, the intensity ratio of I1430/I1075 displayed no apparent changes with other metallic cations (Figure 6C and D).

Figure. 7 The SERS intensity images of functional group at 1430 cm−1 of p−ATP on the AgNPs/PEI/PVA nanofibrous mats in acidic (pH 3.49), neutral (pH 6.07) and alkaline (pH 9.60)


16

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BR buffer solution. (A1, A2 and A3) White light images of the p−ATP (10−5 M) on composite nanofibrous mats with a random selected 20 × 20 µm2 areas. (B1, B2 and B3) SERS intensity maps at 1430 cm−1 of the p−ATP-AgNPs/PEI/PVA nanofibrous mats composed of 400 spectra corresponding to the selected areas shown in A1, A2 and A3. (C1, C2 and C3) The overlay images about the white light images (A1, A2 and A3) and SERS intensity maps (B1, B2 and B3). Laser wavelength: 532 nm; Power: 28 mW; Lens: 50× objective; Acquisition time: 4 s. In addition, in order to support the above mentioned good reproducibility and pH sensing capability, point-by-point Raman mapping at 1430 cm−1 of p−ATP (10−5 M) were recorded at randomly 20 × 20 µm2 selected regions with 400 measured spectra on the AgNPs/PEI/PVA nanofibrous mats soaked into three different BR buffer solutions (acid, neutral and alkaline) (Figure 7). From Figure 7B1, B2 and B3, one observes that the major peak (1430 cm−1) of p−ATP gradually increases with the increase of the pH value. This demonstrates that the neutral and alkaline solution are propitious to the formation of b2 types in p−ATP. From the merged pictures between white light images and mappings (Figure 7C2 and C3), furthermore, the homogeneous distribution of the SERS intensity of the 1430 cm−1 of p−ATP supply strong hints to sustain the foregoing mechanism, and to testify that the AgNPs are uniformly immobilized on the surface of the nanofibers, facilitating the SERS signals with good reproducibility. Ultimately, for embodying the water-stable ability of the as-fabricated AgNPs/PEI/PVA nanofibrous substrate, we have investigated its morphology depending on the different immersion times in acidic (pH 3.38) and alkaline solutions (pH 9.16). It was found that the nanofibers after soaking in aqueous solution still retained an adequate morphologies and maintained the density of AgNPs on the nanofibers (Figure S11 in Supporting Information). Furthermore, the response rate of pH sensing is steady after immersing for 2.0 h in acidic and alkaline mediums (Figure S12 in Supporting Information). This confirms that the nanofibers are water stable and that the assembly of AgNPs via electrostatic interaction onto electrospun nanofibers is a successful and preferable to techniques. pH Sensing in Real Urine Samples. The pH of urine as is one of the physiological parameters that not only reflects the acid-base status of a person, but also can be considered as a vital screening test for the diagnosis of many diseases including renal disease, urinary tract infection, acidosis or alkalosis.52,


53

We have

17


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collected six urine samples of healthy volunteers to test the SERS-based pH sensing method. The normal pH values of urine ranges between 4.6 and 8.0 with an average value of 6.0.52 The comparison between the results from our method and a 510 pH meter are shown in Table 1. It is found that the SERS results are similar to the values obtained by the pH meter and exhibited RSD values with < 3.3 %. Therefore, selecting the pair of bands at 1430 and 1075 cm−1 to calculate the SERS intensity ratio proves to be relevant to monitor pH sensing in real biological samples. Based on our current work, we will combine the overall performance of the AgNPs/PEI/PVA electropsun nanofibrous materials with the SERS-based pH sensing to emphasize the advantages of our method from the following points: (1) SERS pH sensor has some outstanding advantages such a no photobleaching, narrow spectral bands and a high sensitive response for detecting the pH fluctuations; (2) the current method could be applied to the small volume samples; (3) the SERS-based electrospun membrane could be used as teststriplike substrates for pH sensing. Table 1. Measurement of pH in Real Urine Samples a urine

a

SERS analysis

pH meter

samples

mean pH

RSD (%, n = 6)

mean pH

RSD (%, n = 3)

1

6.99 ± 0.21

3.1

6.91 ± 0.12

1.8

2

6.69 ± 0.19

2.8

6.74 ± 0.14

2.1

3

7.24 ± 0.24

3.3

7.16 ± 0.06

0.8

4

6.23 ± 0.14

2.3

6.39 ± 0.08

1.2

5

6.13 ± 0.14

2.2

6.03 ± 0.08

1.2

6

6.98 ± 0.10

1.5

6.82 ± 0.10

1.4

In this work, urine samples were from six healthy volunteers and were directly used for

experiments without any post-processing. The SERS measurements of the pH of the urine samples were taken by the same procedures as that of standard samples. (SERS analysis: mean ± SD, n = 6; pH meter: n = 3, n = number of each sample tested.)

CONCLUSION


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In conclusion, we developed a convenient, facile and cost-effective strategy to prepare the AgNPs/PEI/PVA electrospun nanofibrous membrane with a well-distributed assembly of AgNPs on the PEI/PVA nanofibers held by electrostatic effect. On the basis of this novel composite nanofibers, the as-constructed SERS substrate for Raman probing molecules exhibited excellent enhancement effect and wonderful reproducibility. Additionally, the transformation of p−ATP under different pH conditions could be served as a pH indicator by monitoring the intensity ratio of the b2 mode and a1 mode in p−ATP. The pH sensing process was successfully applied to real urine samples with excellent accuracy. The proposed strategy provides a good platform for further development of highly active SERS substrates by immobilization of noble metal nanoparticles on electrospun nanofibers. Applied to different target analytes, this novel concept will further be widely used in other bio- and chemical sensing applications.

ASSOCIATED CONTENT Supporting Information Additional data and information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the Natural Science Foundation of China (NSFC, 21375109, 21535006), the fund of Chongqing Fundamental and Advanced Research Project (cstc2013jcyjA50008) and the Fundamental Research Funds for the Central Universities (XDJK2016D035). REFERENCES 1. Wu, H.-Y.; Cunningham, B. T., Point-of-Care Detection and Real-Time Monitoring of Intravenously Delivered Drugs via Tubing with an Integrated SERS Sensor. Nanoscale 2014, 6 (10), 5162-5171.


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Table of Contents (TOC)


25

Electrostatic Assemblies of Well-Dispersed AgNPs on the Surface of

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