1. Introduction

Surface-enhanced Raman spectroscopy (SERS) has attracted considerable attention as a powerful tool for ultrasensitive and label-free molecular detection of bacteria [1], biomarkers [2,3], pesticides [4–7], and illegal additives [8–11], and so on [12]. The detection is typically performed on hotspot-rich enhancing substrates that comprises two or three-dimensional (2D or 3D) arrays of densely packed metal nanoparticles [13] or 3D sophisticated metal aggregation on rigid or flexible supports, such as glass [14], aluminum films [15,16], silicon wafer or pyramidal silicon [17,18], PET [19–21], and PDMS [22–24]. However, it is challenging to distinguish and discern components of determinand let alone concentration due to inevitable mutual interference of the Raman spectrum of different components when these SERS-based substrates were applied in real-word scenarios. Great effort has been made to solve this problem. Particularly, the integration of separation capability with Raman detection into a single sensing system has drawn great attention due to its low-cost and effective features. Regrettably, the coupling of SERS with chromatography technology such as gas chromatography [25], thin layer chromatography (TLC) [26,27], high performance liquid chromatography (HPLC) [28,29], etc. has been limited on account of requiring skilled personnel, expensive instrumentation and tedious identification process for different components.

Recently, paper, rich in cellulose fiber networks, can accurately deliver analytes onto the other terminal, allowing for chromatographic extraction or separation without the presence of an auxiliary pump and it has displayed its competence as a potential candidate for SERS application [30–35]. As for the paper-based SERS substrates, the combination of intrinsic deliverable properties with SERS-based signal amplification allows for the detection of complex solution including multi-components at ultralow concentrations. Moreover, flexibility, inexpensive, portability, efficient for sample collection, and readily disposable also contribute to its popularity.

To optimize SERS performance of paper-based SERS substrates, various methods have been used to fabricate metal nanostructures on various papers such as filter paper [36–38], PLLA nanofibrous paper [39], photocopy paper [40,41], and chromatographic paper [42]. Li et al. prepared a paper-based 3D SERS substrate via silver mirror reaction, the detect limit was 10⁻¹¹ for R6G [43]. Rui Zhou et al. patterned Au PNSs on laboratory filter paper by immersing the paper into the Au PNSs solution [44]. Kun Zhang et al. reported paper-based substrates by dip-coating Ag@SiO₂ core/shell nanoparticles into the paper [45].

However, only a moderate sensitivity caused by inhomogeneous and discrete distribution of nanostructures mainly was attributable to unapt fabrication methods [46–49]. No denying that the current situation of only bare noble metal used as enhancing media almost in all these studies indeed faced daunting challenges, such as distortion of SERS spectra arising from interactions between metal and molecules, instability of the metal nanostructure (especially for silver metal), and low reproducibility of the signal intensity induced by photo induced damage. Although the SiO₂ layer could improve the capability, it largely decreased detection sensitivity of plasmonic nanoparticles leading to attend to one thing and lose another. It remains a challenge to develop a sensitive, effective, and reproducible SERS-based paper substrate with an efficient, economical, simple, clean and environmental method.

Considering for the complexity of structure of paper, it is on demand to develop an effective and simple strategy used for growing metallic structures on paper, Rather than conventional fabrication techniques. The integration of silver nanoparticles to 3D substrates by a seeds-induced an in situ growth method has numerous advantages. They are simple, handy reproducible, and inexpensive. Most importantly, they are able to provide the cover of SERS hot spots on any part of the paper not just limited surface deposition. In other words, the in situ growth method can apply to the substrates of any kind of shape. Porous alumina membranes have been successfully inserted Ag nanoparticles with the in situ growth method by Chang et al [50]. Nevertheless, there are few reports on the integrating silver nanoparticles to chromatographic paper by a seeds-induced in situ growth method to construct ultrasensitive 3D SERS-based molecules sensing devices.

Herein, we demonstrated the construction of SERS-based paper test card firstly by a seeds-indued in situ growth method on the crinkly and accidented chromatography paper. Combination of paper with graphene oxide (GO)-veiled Ag nanoparticles (GO@AgNPs) was achieved for on-site, ultrasensitive, reproducible and stable SERS measurement of multiple components. GO has been proved superior chemical stability and bio-compatibility owing to the active oxygen sites [51,52], which have advantages for production of accurate and stable SERS signal, where GO plays three roles at the same time. (1) GO, as the enhancing unit, isolates Ag nanoparticles from external environment not only to avoid the direct contact of metal nanoparticles with the probed molecules, but also to protect the Ag surface from oxidation. (2) GO, as the fluorescence quencher, enables to increase the ratio of Raman signal to background fluorescence. (3) GO, as the molecule enricher, improves the adhesive rate of probed molecules. In this paper card implementation, besides the component in sample mixtures can be efficiently separated from each other based on the polarity differences, extraordinarily, determinand can be converged at the well-designed sharp tip of the card to achieve the concentration effect when laterally flow through the paper in virtue of capillary forces to detect analyte of ultra-low concentration (as low as 10⁻¹⁹ M). Considering its brilliant separation and detection capabilities, low cost, and good reproducibility, the established SERS-based 3D sensing device can serve as a dependable tool for real sample detection and analysis.

2. Experimental section

2.1 Reagents and materials

All chemicals were of analytical grade and were used without further purification. Chromatography paper (Whatman1CHR No. 3001-917), aqueous solution of acidic stannous chloride, AR silver nitrate, ascorbic acid, rhodamine 6G (R6G), crystal violet (CV), toluidine blue (TB), ethanol (99.7%), acetone, GO aqueous dispersion were all purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Deionized water was used through the experiments.

2.2 Fabrication of the paper sensor based on GO@AgNPs

The chromatography paper strip successively was immersed in acidic aqueous solution of SnCl₂ (0.2M) and AgNO₃ solution (0.2M) for two minutes. Then it was rinsed by pure water and acetone respectively. This process was repeated three times to provide a high density distribution of silver seeds on the chromatography paper strip. Subsequently, the silver seeds then grew into Ag nanoparticles by immersing the paper into the mixed solution of AgNO₃ and ascorbic acid for a certain time, followed by air-drying. Finally, The GO was impregnated into chromatographic paper strip by immersing the paper strip into GO aqueous dispersion (0.5mg/ml) for moment, followed by spin-coating and air-drying. This process was carried out three times to improve the GO coverage rate.

2.3 Numerical analysis of electric field distribution

The electric field properties from the silver nanoparticles on cellulose fibers were numerically calculated by commercial COMSOL software. The size and gap distance of the silver nanoparticles were extracted by reference to scanning electron microscope (SEM) images, and each silver nanoparticle was considered a spherical shape for the 3D COMSOL analysis

2.4 Characterization

Scanning electron microscope (SEM, Zeiss Gemini Ultra-55) detection was conducted to elucidate morphologies such as the size and degree of aggregation of the Ag. A Rigaku D/ Max-gA rotating-anode X-ray diffractometer was used to determine the phase and composition of the products. Raman spectra were collected using Horiba HR Evolution 800 excitation with 532 nm Ar excitation laser. The size of laser beam focused on sample was about 1 μm, and the effective power of the laser source was kept at 0.48 mW for R6G and CV molecules. The system was connected to a microscope, and the laser light was coupled through an objective lens of 50 × , which was used to excite the samples as well as collect the laser light. The SERS spectra on display were the average value of spectra from three different positions. A PerkinElmer Lambda 750 UV-visible spectrophotometer was applied to measure the UV-vis extinction spectra of metal material. The fluorescence spectra of R6G molecules on paper tip were characterized with an inverted confocal laser scanning microscope by sequentially imaging the optical sections. Transmission electron microscopy (TEM) images of GO were obtained by a transmission electron microscopy system (JEOL, JEM-2100).

2.5 Separation and preconcentration process

First, samples were spotted near the edge of the substrate by capillary tube with a volume of 0.1 uL as indicated in Fig. 1. After droplets were dried under ambient conditions, a small amount of 99.7% ethanol (3-5ml) as a mobile phase was poured into a glass beaker or wide-mouth bottle which was then covered with a glass lid or glass stopper to presaturate the beaker volume with ethanol vapor which needed about 15 min. Finally, the paper substrate was then carefully placed upright into the mobile ethanol in glass beaker to separate mixture solution or concentrate the molecules into sharp tip of the paper. The sample spots on the paper were in proximity to but did not contact the mobile ethanol meniscus. During the development process, the beaker remained covered to minimize mobile ethanol transpiration from the paper substrates.

 

Fig. 1 Schematic representation of the paper-SERS process.

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3. Results and discussions

As mentioned earlier, developing a paper-based high-performance SERS sensor substrate with an adaptive method is very urgent. For this purpose, we have successfully assembled GO-veiled Ag Nanoparticles composite on the paper to further develop paper-based SERS. In this study, Laboratory chromatographic paper (Whatman grade 1) composing of microscale (∼18 μm) cellulose fibers interweaving with each other to form hierarchical and porous configuration was used. Seeds-induced in situ growth process consisting of electroless-deposition of Ag seeds and in situ growth of silver nanoparticles was applied to achieve the coupling of uniform plasmon active Ag layer with the 3D paper and this method was feasible to achieve the 3D wrapping of cellulose fibers, as illustrated in Fig. 1. The AgNPs- deposited paper had a brilliant dark appearance. After that, we just simply immersed the paper into the GO dispersion liquid preparing GO shrouded paper substrate by the dip-coating method. After the AgNPs- deposited paper was covered by the GO layer, the surface color of paper changed a little.

Seeds-induced in situ growth method used here worked very well for forming high- density Ag nanoparticles existing throughout the any corner of the 3D paper. Firstly, Silver seeds were immobilized on the paper by a simultaneous reduction and substitution process as shown in Fig. 1. During the process, the Sn²⁺ would automatically get deposited on the cellulose fibers when paper was immersed in an acidic SnCl₂ solution (0.02M), which was the first. After that, all positions of Sn⁺ on the paper were replaced by silver nanoparticles with small size by virtue of a simultaneous reduction and substitution process after soaking the paper into aqueous AgNO₃ solution, which extended all over the cellulose fibers. The electroless-deposition of Ag seeds on the surface of the paper was completed by the following reduction reaction 1.

To increase the density of Ag seeds and form abundant hot spots on the paper, the cycle of Ag seeds deposition was carried out three times [Fig. 2(a)]. Then, in situ growth process was employed to regrow silver seed to assemble properly-sized silver nanoparticles on the cellulose fibers, which was achieved by exposing the paper to the growth solution consisting of 100 mL of 0.01M AgNO₃ and 50 mL of 0.1 M ascorbic acid (AA) shown in Fig. 2(b), and larger and more highly-concentrated silver nanoparticles were obtained on the basis of the as-formed silver seeds on the surface of the cellulose fibers attributing to a heterogeneous nucleation and growth mechanism.

 

Fig. 2 (a) SEM images of silver seeds. (b) SEM image of in situ growth of silver nanoparticles from electroless-deposited seeds in paper for 1 min growth time. (c) and (d) SEM images of in situ growth of silver nanoparticles from electroless-deposited seeds in paper for 3 min growth time under different magnification. (e) Raman spectra of R6G on paper at various growing periods, particularly, the lines marked by “8min-1” and “8min-2” display the SERS performance of Ag nanoparticles and Ag aggregations on the paper after 8-min growth respectively. (f) Relative Raman intensity of 613 cm⁻¹ at different growth-time.

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As known, only the diffusion of growing species in the growth media to the surface of the growing particle limits the homogeneous growth process of nanoparticle on the paper surface. In this way, the paper deposited by the preliminary Ag seeded can well control the regrowth step of silver nanoparticles on the paper surface on the basis of the concentration and total quantity of growth species faced by the growing particle on the paper. Here, the actual size of the silver nanoparticles grown on the paper surface in relation with the growth time was studied, which was illustrated in Fig. 9 and Figs. 2(a)–2(d). Possibly attributing to the largest concentration existing in the beginning of the whole growth process, the increase of silver nanoparticles in size was quick at the beginning when the paper substrate was exposed to the nanoparticle growth solution, which was monitored by SEM images shown in Fig. 9(a). It was observed that a gradual increase in the size of the nanoparticles was formed on the paper when more time for silver nanoparticle growth was allowed in mostly within the initial 5-6 min [Fig. 9(b)]. With time further increased, the size of nanoparticles almost kept unchanged due to the running out of growth media [the inset of Fig. 9(c)]. But, as time further went, Ag aggregations, the reaction product suspending in the solution, gradually sank down on the surface of paper as shown in Fig. 9(d). These Ag aggregations damaged the homogeneity of substrate and decreased the SERS performance, which brought nothing but harm to the substrate.

To investigate the SERS performance of silver nanoparticles decorating the paper at various in situ growth times, we collected their SERS response to 10⁻⁶ M R6G [Figs. 2(e) and 2(f)], after each SERS substrate was soaked in 1 × 10⁻⁶ M Rhodamine 6G (R6G) aqueous solution of for three hours for the deposition of R6G molecules. Obviously, in one minute, the size of Ag nanoparticles increased hugely, meaning the large improvement of SERS performance of substrates. And the substrate containing silver nanoparticles grown for 2-3 min provided the largest SERS enhancement. When the growth time reached to and even more than 6min, the average space between Ag nanoparticles increased so large that missing the optimal state. It is well known that the size and level of aggregation of spherical metal nanoparticle play decisive roles in their plasmon resonance properties. As shown in Figs. 2(c) and 2(d), the aggregated silver nanoparticles of average 50 nm after 3-min growth were proved to be the optimum conditions for the best SERS enhancement performance.

Moreover, in the present preparation process, it was found that the molar ratio of the reactants strongly affected the obtained spatial arrangement of the silver nanostructures including the size and interval of Ag nanoparticles. Silver nanoparticles with average diameters and spacing of 50, 7 nm were formed on the paper substrate at a high molar ratio of 5, as shown in Fig. 3. A decrease in the molar ratio of AA to Ag + from 5 to 1 resulted in larger size and larger interval-distance of Ag nanoparticles with approximately 100nm, 30nm [Fig. 3(b)]. When the molar ratio further decreased from 1 to 0.5, a large number of very small silver nanoparticles with just 30 nm in size were formed. Inevitable, some Ag nanoparticles aggregations randomly distributed in the surface of the paper substrates [Fig. 3(c)]. The spatial distributions of the electromagnetic field intensity of Ag nanoparticles produced under molar ratio shown in Figs. 3(e) and 3(f) was simulated by the COMSOL software corresponding to the sample substrates displayed in Figs. 3(a)–3(c) respectively. The 532nm laser was perpendicularly incident the sample substrates. It is known that the electric field enhancement around the nanoparticles greatly affects its SERS activity. Here, the magnitude of electrical field significantly locates at the edge of nanoparticles, especially in the gap region with Ag nanoparticles displayed in Fig. 3(a) is obviously larger than that with Ag nanoparticles exhibited in Figs. 3(b) and 3(c).

 

Fig. 3 Typical SEM images of the products on the paper prepared at molar ratios of AA to Ag + of (a) 5, (b) 1, (c) 0.5. (e)-(g) are the electric field distribution images of the products corresponding to (a)-(c). (d) SEM images of GO@AgNPs on the paper prepared at molar ratios of AA to Ag + of 5. (h) is the electric field distribution images of GO@AgNPs corresponding to (d).

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The AgNPs-decorated paper substrate was further developed to be GO-shell isolated SERS substrate by the dip-coating method, which improve its SERS performance in terms of reproducibility. A Hummers method was applied to synthesize 0.25mg/ml GO dispersion solution. Figure 4(a) displays the TEM image of the GO membranes that were used in this study. With the virtue of the nature adsorptive property of paper substrate, the dip-coating method used was feasible and appropriate to achieve the coverage of GO membranes on AgNPs@paper substrates. Moreover, in order to improve the coverage rate of GO films, the dip-coating process was repeated three times. Compared with the substrates with only Ag nanoparticles, as shown in Figs. 4(b) and 4(c), AgNPs@paper surface after covered by GO films became rough and fuzzy, and some wrinkles of GO films were faintly visible on it, which is unavoidable and insignificant in the dip-coating process. From the SEM images, over 80% area of AgNPs@paper substrates was wrapped by GO films successfully. In order to provide a strong support of successful combination of GO and AgNPs@paper substrates, the Raman spectra were randomly collected from the AgNPs@paper wrapped by GO films (GO@AgNPs@paper substrates) shown in Fig. 4(d). Obviously, AgNPs@paper substrate was successfully covered by the GO film. The spatial distribution of the electromagnetic field intensity of GO@AgNPs under molar ratio of 5 [Fig. 3(h)] was also simulated by the COMSOL software corresponding to the sample substrate displayed in Fig. 3(d). It is observed that electromagnetic field intensity of GO@AgNPs is almost the same with that of AgNPs. So in theory, for the thin GO film, it has small effect on the electromagnetic field intensity of AgNPs. The experimental results shown in Figs. 4(e) and 4(f) agree with the above theoretical results. The cover of GO film leads to a modest decline in the Raman intensity of paper substrate. Using 10⁻⁶M R6G as the probe molecules, the time-dependent experiments were carried out to compare the signal stability from AgNPs@paper with that from GO@AgNPs@paper. As shown in Figs. 4(e) and 4(f), the SERS intensity of R6G on the bare AgNPs@paper decayed largely after seven-day storage in ambient conditions, while the signals of R6G on GO@AgNPs@paper still remained around 80% of its original SERS activity, displaying a remarkable advantage in terms of the temporal stability. The better stability performance of GO@AgNPs@paper substrates attributed to the very stable chemical properties of GO shell.

 

Fig. 4 (a) TEM of the obtained GO. (b) Raman spectrum collected from the GO@AgNPs@paper substrate. (c) The SEM image of the GO@AgNPs@paper sample. (d) Partial details of the SEM image shown in Fig. 4(c). (e) and (f) SERS spectra of R6G measured from the surface of AgNPs@paper and GO@AgNPs@paper before and after storage for 7 days respectively.

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Besides the time stability, both high sensitivity and reproducibility are also very important criterion for judging an excellent SERS substrate. In order to further evaluate the quantitative detection range of the established paper substrate, we measured R6G molecules in the region from 500 to 1800 cm⁻¹ at concentrations ranging from 10 nM to 10 aM shown in Fig. 5. Quantitative detection in R6G concentrations ranging from 10 nM down to 10 fM was achieved in our study. Within this concentration range, the SERS intensity at 1362 cm⁻¹ (C–C stretching mode of R6G) showed a good linear SERS dependence with the R6G concentration with the high coefficient of determination (R²) (0.967). As the concentration of R6G molecules further decreased down to 10 aM (10⁻¹⁷ M), our as-prepared paper substrate still could detect the SERS signals of R6G with strong intensity at random sites over the paper. Moreover, the CV molecules also were used as probe molecules to further examine the SERS performance of GO@AgNPs@paper substrates to different colouring agent. Similarly, the substrate still displayed good linear SERS dependence with the CV concentration within the concentration range from 10⁻⁶ M to 10⁻¹² M at the 911 cm⁻¹ with the high coefficient of determination (R²) (0.993). Particularly, as low as 10⁻²⁰M CV molecules still was traced by the GO@AgNPs@paper substrate through soaking the paper substrate into the 1mL 10⁻²⁰ M CV solution in spite of the strong fluorescence background. To our best knowledge, this excellent detection ability is far beyond other paper-based SERS substrates. The superior SERS enhancement performance was possibly ascribed to the combination of the close proximity induced high-intensity hot spots benefiting from the seeds-induced in situ growth mode and the improved molecular adsorption of the flexible GO@AgNPs@paper substrates.

 

Fig.5 (a) and (b) Raman spectra of R6G with different concentrations from 10⁻⁸ to10⁻¹⁷M. (c) Raman intensity of R6G at 1,362 cm⁻¹ as a function of the R6G concentration. (d) and (e) Raman spectra of CV with different concentrations from 10⁻⁶ to10⁻²⁰ M. (f) Raman intensity of CV at 911cm⁻¹ as a function of the CV concentration

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The reproducibility of the SERS signals collected from randomly selected positions over the GO@AgNPs covered paper surface with over 2 mm space interval was evaluated. Figure 6(a) shows the SERS spectra of R6G molecules at 10⁻⁶ M from 10 different locations in the same GO@AgNPs@paper substrate. The profile of SERS spectra of R6G collected from these positions was very similar both the locations of major Raman peaks and the Raman intensity, indicating high reproducibility. The spot-to-spot SERS intensity variation of the characteristic 612 cm⁻¹ peak was used for the quantitative evaluation of the signal fluctuation [Fig. 6(b)]. Formula giving the maximum intensity deviation about the SERS spectra has been reported as follows:

 

Fig. 6 (a) SERS spectra of R6G at 10⁻⁶M were collected from10 randomly selected spots from the paper substrate. (b) Intensity distribution of the 613 cm⁻¹ peak in the 10 spectra from a same paper substrate. (c) SERS spectra of R6G at 10⁻⁷M were collected from10 paper substrates in different batches. (d) Intensity distribution of the 613 cm⁻¹ peak in the 10 spectra from 10 paper substrates in different batches.

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Considering the complicated construction of nature paper substrates, a relative standard deviation (RSD) of 22.7% for Raman intensities at different positions was obtained encouragingly, revealing relatively excellent reproducibility of GO@AgNPs@paper substrates. Moreover, SERS signals of 10⁻⁷M R6G molecules on different GO@AgNPs@paper substrates in various batches also were collected shown in Fig. 6(c). The substrate-to-substrate SERS signal intensity fluctuation variation of characteristic peak at 612 cm⁻¹ was very small revealed by the narrow shaded area (green color) [Fig. 6(d)]. All the intensities of the peak of 612 cm⁻¹ lie within a 17.9% floating range. The reproducibility of SERS signal from GO@AgNPs@paper substrate has suited the requirements for quantitative measurements according to the existing scientific standards about evaluating the reproducibility of SERS substrates suggested by Natan [53]. The high reproducibility obtained from GO@AgNPs@paper substrates can be attributed to at least two aspects: First, the seeds-induced in situ growth method was successfully applied to construct uniform and high-intensity hot spots on 3D substrates such as paper. Second, the GO@AgNPs@paper surface can effectively and homogenously capture the interested molecules, reducing the signal blinking induced by the drift of analyte molecules on SERS-active surfaces under excitation laser.

The high sensitivity, stability, and reproducibility of SERS signal from probe molecules detection indicate that GO@AgNPs@paper substrates display great potential for paper-based practical applications. It is well known that the abundant porosity and patterned paper channels allow for the transmitting or separation of analyte molecules using capillary action (e.g. without external pumping). Thereby, the application of so penny worthy paper has been extensively and profoundly explored in many fields such as blood, urine, and cell extraction ever since this technology springs up [54].

There, based on different affinities of CV and TB for the stationary and mobile phases which were caused by polarity difference of two molecules, a mixture of CV and TB was used to further investigate the separation ability of GO@AgNPs@paper platform. The mixed solution of CV (10⁻⁵ M/ 1mL) and TB (10⁻⁵ M/1mL) with a volume of 0.1 mL was pipetted on the main body of the paper and then dried under ambient conditions. Using 99.5% methanol as the mobile phase, CV and TB were successfully separated on the paper based ascending chromatography as shown in Fig. 7. Chromatographic process was developed in an air tight bottle. The absorbance spectra of the analyte molecules were then measured at different positions (chromatograms) over paper-based substrate. The absorption bands of TB and CV were clearly distinguished at positions 1 and 2, [Fig. 7(b)] respectively. In addition, the SERS spectra of the individual analytes also confirm the molecular fingerprints at each chromatogram Fig. 7(d). As displayed in Fig. 7(a), SERS spectra of CV molecules in different locations were collected. As can be seen, the CV molecules were dispersed well at the paper surface.

 

Fig. 7 (a) SERS spectrum of CV collected from different locations of paper corresponding to Fig. 7(c). (b) Absorption spectra of TB and CV measured respectively from the paper surface as shown in Fig. 7(c). (c) Photographs of the paper strip after the chromatographic separation. (d) SERS spectrum measured from the paper strip after separation.

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Furthermore, the nature capillary force of chromatographic paper, which can deliver the sample molecules on the paper from one side to another, inspires us to fabricate a molecules-concentration device based on the paper. As shown in Fig. 8(a), the paper strip was cut into a tree-like profile with a nearly 20° sharp tip down to single fibers. The entire strip was 20 mm × 80 mm (maximum width × length). This design of head light and feet heavy not only is convenient for sample loading and migration, more importantly, but also can increase the local concentration of the analyte of interest significantly when it is delivered to the triangular-shaped sharp point, which can remarkably improve the detection sensitivity of paper-based analytical devices. R6G as a model analyte was used to explore the preconcentration effect of paper device on the limit of detection (LOD). A 50 uL aliquot of R6G spotted on the main body near the lower edge of the triangle was driven to the paper card tip by using ethanol as a mobile phase after eight-hour chromatographic process demonstrated in Fig. 8(b). The fluorescence image of enlarged paper tip after the preconcentration was shown on the right of Fig. 8(b). It was obvious that the dispersed R6G molecules on the paper were gathered in the tip after the preconcentration process.

 

Fig. 8 (a) Digital photographs showing R6G dye before chromatographic preconcentration. (b) Digital photograph and fluorescence image of the paper card after chromatographic preconcentration. (b) SERS spectra collected from the tip of paper after preconcentration.

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The SERS spectrum of R6G on the tip of the paper was collected after drying in air. As shown in Fig. 8(c), GO@AgNPs@paper based substrates display great potential in gathering the molecules in solution into a smaller region to achieve the preconcentration effect and R6G molecules with the concentration as low as 10⁻¹⁹ M was still clearly detectable, which corresponds to only several molecules existing in the sample solution. In addition to well keeping up the nature transmission ability of paper, the excellent preconcentration performance of GO@AgNPs@paper based substrates can be assigned to its high-sensitive and reproducible SERS behavior, which potentially will be an epoch-making event in the developing history of SERS-based sensor.

4. Conclusions

In summary, we design a multifunctional SERS-based paper strip based on GO-veiled Ag nanoparticles with high SERS performance in terms of sensitivity, stability, and reproducibility for the first time. The paper strip displayed excellent SERS activity with a minimum detected concentration of 10⁻¹⁷M and 10⁻²⁰M for R6G and CV molecules respectively. Moreover, in virtue of inherent ability of paper to separate complex solution, the paper strip enables the excellent direct analysis and label-free detection of diverse molecules in mixture samples. Furthermore, the well-designed paper strip with sharp tip can be used as an integrated platform allowing as low as 10⁻¹⁹M R6G ultra-sensitive detection from 50uL samples, which greatly improves the detection ability of SERS-based paper substrate.

Appendix

 

Fig. 9 SEM images of time dependent in situ growth of silver nanoparticles from electroless-deposited seeds in paper. (a) 3 min, (b) 5min, (c) 8 min and (d) 10 min of growth time of silver nanoparticles from electroless-deposited silver seed in paper.

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Funding

National Natural Science Foundation of China (NSFC) (11474187, 11274204, 61205174); Shandong Provincial Natural Science Foundation, China (ZR2016AM19).

Acknowledgments

Great thanks to Professor Yanyan Huo for her helpful suggestions in our COMSOL analysis. Thanks to Professor Chuansong Chen for his great help in the manuscript writing.

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