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Ag-nanoparticles on UF-microsphere as an ultrasensitive SERS substrate with unique features for rhodamine 6G detection
Talanta 146 (2016) 533–539
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Ag-nanoparticles on UF-microsphere as an ultrasensitive SERS substrate with unique features for rhodamine 6G detection Zhixian Hao n, Mulati Mansuer, Yuqing Guo, Zhirong Zhu, Xiaogang Wang Shanghai Key Laboratory of Chemical Assessment and Sustainability, Department of Chemistry, Tongji University, Shanghai 200092, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 July 2015 Received in revised form 7 September 2015 Accepted 10 September 2015 Available online 12 September 2015
Urea and formaldehyde (UF) microsphere (MS) adsorbing Ag nanoparticles (NPs) was employed as a surface enhanced Raman scattering (SERS) substrate for rhodamine 6G (R6G) detection. The UF MSs and citrate-reduced Ag colloid supplying Ag NPs are synthesized separately and all the subsequent fabrication procedure is then implemented within 2 mL centrifuge tube. Influences of the composition and drying temperature of the UF MSs and the drying method and modification of AgNP/UFMS on the final SERS performance have first been reported. Excess formaldehyde useful in the formation of UF MSs again plays an important role in the SERS detection. Some interesting phenomena in the approach, such as swelling/deswelling of UF MSs and R6G diffusion within hydrophilic environment of UF MSs, are found to be of variable factors affecting the SERS performance. The substrate AgNP/UFMS confidently achieves a detection limit of 10 13 M R6G and can be used as a simple and effective platform in the SERS spectroscopy. & 2015 Elsevier B.V. All rights reserved.
Keywords: UF microsphere SERS Ag nanoparticles Swelling Hydrophilic
1. Introduction SERS spectroscopy [1,2] has gone through an impressive resurgence in the last three decades, of which the molecular fingerprint specificity with potential single-molecule sensitivity [3] is extremely attractive to chemical and biological detections [4]. When adsorbed on rough metal surfaces or on aggregates of metal NPs, some of analytes under laser excitation have been found to give rise to enhanced Raman signals even by a million-fold or more [5]. Therefore, building metal nanostructures with SERS-active “hot spots” [1,2] has become a significant procedure for SERS detection. Various fantastic Ag/Au nanostructures, such as nanoflowers [6–8], nano-rods [9–11], nano-sheets [12,13], and nanoarrays [14–17], are incredibly fabricated by means of some bottomup assemblies or top-down designs. However, it is difficult to accept these approaches in routine detection because of their complicated fabrication procedures, their high cost, or some inherent defects in their storage. A key practice in SERS detection is to draw adjacent metal NPs at a sufficiently close distance so as to gain SERS-active “hot spots” [18,19] and herein some organic crosslinkers always act as a controlling role. For example, two amino groups in hexamethylene Abbreviations: UF, urea and formaldehyde; MS, microsphere; NP, nanoparticle; SERS, surface enhanced Raman scattering; R6G, rhodamine 6G; AgNP, Ag nanoparticle; UFMS, urea and formaldehyde microsphere n Corresponding author. E-mail address: [email protected] (Z. Hao). http://dx.doi.org/10.1016/j.talanta.2015.09.024 0039-9140/& 2015 Elsevier B.V. All rights reserved.
diamine [20] are responsible for bonding Ag NPs and the hexane chain to hold them at the distance itself. Supporting Ag NPs on functionalized surfaces of some silicon/silica materials is a general accepted method in fabrication of SERS substrate [21–23] and it was confirmed that hydrophilic groups grafted on the surface, such as –NH2 and –SH, can bound metal NPs, but hydrophobic ones, such as –CH3 and –PPh2, cannot [21]. The polymer with more than two short branched chains attaching metal affinity groups at each end is a class of significant crosslinkers in fabrication of SERS substrate. These branched chains working as multi-arms with hydrophilic anchors make assembly of some novel NP architectures much more flexible [24–27]. A recent study [28] verified that the metal NPs can be assembled into a 1-D line, a 2-D plane or a 3-D sphere by mediating the number of chains in a hyperbranched molecule and these NP architectures present a gradually increasing SERS response with the dimensions of themselves increasing, similar to the performance of a gradually enlarged NP cluster [29]. A further work introduces metal NP aggregates into solid/gel polymers or biopolymers with hydrophilic groups by various bottom-up approaches. A uniform and dense Au nano-rod coat on polycaprolactone fibers was achieved through electrospinning technique combining with traditional polyelectrolyte layer-bylayer deposition [30]. Another work grafted fine 3-D Ag nanosheets on polyamide fibers based on electrospinning technique herein uniting with cultivation of Ag nano-sheets from Au NP seeds born in the fibers in situ [31]. Composition of Ag NP spheres within “high-aspect-ratio” benzene tetracarboxylic acid-doped
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polyaniline fiber was recently reported based on a simple solutiondipping method with the fiber itself as both the reducing agent and the carrier [32], which achieves a R6G detection limit equal to 10 10 M. An extreme approach traps metal NPs into poly(N-isopropylacrylamide) microgel [33,34], polyacrylamide particles [35], or agarose gel [36] while swelling/deswelling of the polymer is designed to control the gap size between the metal NPs. Although dense hot spots can be created with numerous hydrophilic groups anchoring at metal NPs from different 3-D points, both an additional swelling/deswelling step and a bottom-up assembly of metal NPs makes these approaches complicated in routine analysis. In previous papers [37,38] we had reported the precipitation reaction of UF MSs and the role of excess monomer in growth of UF deposits. In this one the UF MS adsorbing Ag NPs based on an impregnation method is employed as a SERS substrate. The UF MSs and Ag NP colloidal solution could be acquired and stored separately in a manner of large quantity. The fabrication, incubation and separation of AgNP/UFMS can be simply carried out in mL scale under routine laboratory conditions, while some variable factors in this approach were first described with some appropriate discussion. R6G chosen in this paper is a common biological dye, known to be able to interact with Ag NPs. It is usually employed as a SERS reporter with a larger Raman cross-section in bioassays or for evaluation of SERS substrates.
2. Experimental 2.1. Preparations of UF MSs and Ag NP solution Synthesis of UF MSs was reported previously [37,38]. In simple terms, 2.25 g urea, variable volume of formaldehyde solution (38 wt%) and 40.0 mL deionized water reacted at 28.0 °C for 30 min under catalysis of 0.200 mL hydrochloric acid (37 wt%). The UF MSs was sequentially separated in a Buchner funnel, washed with water and ethanol two times, respectively, dried for 12 h in ambient environment, and then stored in a dessicator for use after dried in oven for 3 h. Herein, the drying temperature of UF MSs and the molar ratio between formaldehyde and urea used in the synthesis reaction were designed as variable factors to investigate the final SERS performance. Ag NP colloidal solution was prepared following a standard procedure [39] with a little of modification. After 5 mL Na3C6H5O7 solution added into 95 mL boiling AgNO3 solution, the beaker was sealed with a piece of plastic paper and held boiling under electromagnetic stirring for 30 min. The initial concentrations of Na3C6H5O7 and AgNO3 added were equal to 3.0 10 3 and 2.0 10 3 M, respectively. The water used in preparations of the Ag NP colloidal solution and the subsequent SERS substrate had an electrical conductivity of 1.37 μS/cm. All chemicals were of analytical grade and used without further purification. 2.2. Fabrication, treatment and incubation of the SERS substrate Before use, Ag NPs within a 2 mL colloidal solution spanning a range of centrifugal rate from 2000 to 6000 rpm was collected and re-dispersed in water to the initial volume, where both the precipitate at the 2000 rpm for 5 min and supernatant at the 6000 rpm for the same period were discarded. Meanwhile, the spanning range of centrifugal rate was changed to investigate its influence on the final SERS performance. Next, 50 mg UF MSs was added into the graded Ag NP solution and separated at 6000 rpm after 60 min impregnation. The AgNP/UFMS was then treated with a 1.0 10 3 M KCl solution or some other inorganic salt solutions to investigate their effect on the SERS performance. Incubation of the AgNP/UFMS after separation was carried out in situ within
2 mL 1.0 10 7 M R6G for 1 h and separated at 6000 rpm following each step. Herein, both the period of incubation and concentrations of R6G were arranged as variable factors to investigate their effect on the SERS performance. 2.3. SERS detection and characteristics of the SERS substrate A little of the incubated AgNP/UFMS was transferred onto a glass slide and dried at ambient temperature in a lab environment with a relative humidity of 45–65% or in a dessicator for 12 h for Raman spectroscopy interrogation. Before the transfer the separated AgNP/UFMS sample in the tube was re-dispersed in 0.3 mL ethanol while the glass slide was treated in ethanol for 10 min under ultrasonic vibration and then a drop of the mixture was used. In order to recover the SERS performance obstructed in a dessicator drying process, the sample was treated for 12 h within saturated water vapor in a dessicator, equilibrated with some liquid water at bottom instead of the silica gel. Raman spectrum was measured on a confocal microscope Raman spectrometer (Invia Reflex Renishaw) with a 514 nm laser as excitation. The laser beam with a spot diameter of ∼1 μm was focused on the surface of sample with an accumulation time of 10 s at a laser power of 20 mW using a 50 microscope objective. Meanwhile the laser power was allowed to choose a less value available to the SERS response. Morphology and element analysis of UF MS or SERS substrate were investigated using a scanning electron microscope (SEM) (Quanta FEG 250, FEI Company) operated at 10.0 kV and an energy dispersive X-Ray spectroscopy (EDS) (Genesis APEX 2, EDAX Company). A little of sample dispersed in ethanol was dropped on a coverslip for the followed measurement. In order to investigate the swelling/deswelling of UF MSs after impregnation or drying process, more than 80 SEM images of UF MSs in a target sample was taken for the statistic average diameter.
3. Results and discussion 3.1. UF MSs adsorbing Ag NPs as SERS substrate Fabrication of SERS substrate AgNP/UFMS was implemented within 2 mL centrifuge tubes. 5.0 mg UF MSs was sequentially impregnated in a Ag NP colloidal solution for 1 h, modified in a 10 3 M KCl solution and incubated in a 10 7 M R6G solution for 1 h, while a centrifugation process at a centrifugal rate of 6000 rpm for 5 min followed each of the steps. A little of the sample was then transferred onto a glass slide and dried in a dessicator for SERS measurement or in a lab environment, as shown in Fig. 1. Ag NP colloidal solution used in the above fabrication presented particles mixed with a small quantity of nano-rods (see SEM image Fig. 1c), which had a maximum absorption around 445 nm, as shown in Fig. S1 (Supplementary material), and a white color instead of a deep grey one. The colloidal solution can be optimized using variable rate range in its centrifugation. Influence of the rate range on SERS performance is shown in Fig. S2, where NP aggregates separated at a low centrifugal rate obviously had a high SERS enhancement, due to large Ag NP aggregates obtained at the low centrifugal rate [29]. 3.2. Influence of the excess formaldehyde on SERS performance It has been shown that the lamellar crystallinity and molecular structure of UF MSs strongly depend on some of variable factors in the synthesis itself, such as the acidity, reactant amounts and the U/F molar ratio. The excess formaldehyde, with respect to the urea
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Fig. 1. Schematic for a simple fabrication procedure for SERS substrate. The U1F5 MSs (a) is firstly impregnated in a Ag NP solution, modified in an inorganic salt solution and incubated in a R6G solution, while a centrifugation step follows each of the above ones. The incubated substrate (b) is then transferred onto a glass slide from the centrifuge tube and dried for SERS detection. Therein, the Ag NPs containing a small amount of nano-rods were randomly dispersed on the surface of a U1F5 MS (c) and the SERS band at Raman shift of 1650 cm 1 collected on the AgNP/U1F5 reached an intensity value more than 5 104 cts after it was incubated within a 10 7 M R6G solution.
used [38], plays an important role in the synthesis reaction of UF MSs. Some excess formaldehyde will finally bond at the terminals of polymer molecules, such as existing in the form of linear molecule HOCH2[HNCONHCH2]nOH, or enrich on the surfaces of UF MSs as a plenty of terminal hydroxymethyl groups –CH2OH. Some of further information on excess formaldehyde is shown in Fig. S3. It is just the variability of excess formaldehyde that allows us to tune the average size of UF molecules or UF MSs [38]. In order to obtain a general understanding to the role of excess formaldehyde in the SERS detection, a set of UF MS samples were firstly synthesized with variable excess formaldehyde at a constant quantity of urea, then dried at 100 °C for 3 h and fabricated into substrate samples. Influence of the excess formaldehyde on the SERS performance is displayed in Fig. 2. The SERS response of R6G on sample AgNP/U1F1 was faint (see Fig. 2a), but the SERS signals on other samples gained intensity gradually with the excess formaldehyde increased in the synthesis of the UF MSs.
The AgNP/U1F5 even presented the most intense SERS signal, reaching to 2.3 104 cts at 614 cm 1 more than 10 times the intensity measured on AgNP/U1F1 (see Fig. S4). The fact that an excellent SERS performance was provided by the substrate originating from excess formaldehyde implies that construction of SERS hot spots could be strongly improved by the plentiful –CH2OH groups (or others with a high oxygen content) enriching on the UF MSs. 3.3. Influence of the drying temperature of UFMS on SERS performance Drying temperature of the UF MSs incorporated in AgNP/UFMS is another factor impacting on the final SERS performance. Pure U1F5 MSs just showed a flat Raman background close to the spectral baseline after treated at different temperature (see Fig. 3a–d). However, the SERS signal collected from R6G incubated
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Raman shift [cm-1] Fig. 2. SERS spectra measured on AgNP/UFMS at 1.0% laser power after incubation within a 1.0 10 7 M R6G solution for 2 h. The UF MSs incorporated in the samples were synthesized with formaldehyde amount increasing as a set of U:F molar ratios equal to 1:1 (a), 1:2 (b), 1:5 (c), 1:7 (d), and 1:9 (e). SERS performances on the samples originating from excess formaldehyde (b–e) were more favorable than that on AgNP/U1F1 (a) in which the U1F1 MSs incorporated was considered having a proximately linear molecular structure (HO[CH2NHCONH]nH) [38].
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AgNP/UFMS gained intensity gradually with the drying temperature of the incorporated UF MSs increased from ambient temperature to 100 °C (see Fig. 3e–g). This gradually improved SERS enhancement thoroughly collapsed if the incorporated UF MSs was pretreated at 150 °C for 3 h (see Fig. 3h). If carefully distinguishing the enlarged Raman spectrum measured on the UF MSs treated excessively (see Fig. S5d) from the others (see Fig. S5a–c), a weaker background was found. UF MSs is hydrophilic and was found dehydrated in less than 100 °C [40], while a somewhat low crystallinity was observed as it was treated at 100 °C [40]. These changes are accelerated at 150 °C that should be unfavorable to the SERS analysis.
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Raman shift [cm-1] Fig. 3. Raman spectra collected on U1F5 (a–d) and AgNP/U1F5 (e–h). Before use the U1F5 employed was dried at various temperatures, equal to ambient temperature for 12 h (a, e) and to 50 °C (b, f), 100 °C (c, g), and 150 °C (d, h) for 3 h.
There exist even more complex details in the drying process of R6G-incubated AgNP/UFMS, as shown in Fig. 4. Following an incubation step in a 1.0 10 7 M R6G solution for 1 h, 6 h, or 12 h, a lab environment or a dessicator was adopted to implement the drying process of SERS substrate. An intense R6G SERS signal was observed when the substrate had been dried in a lab environment for 12 h following its incubation for 1 h (see Fig. 4a) and then dropped gradually with the incubation time prolonged from 1 h to 12 h (see Fig. 4a–c). If the substrate had been dried in a dessicator, on the contrary, a faint SERS signal collected from the sample incubated for 1 h (see Fig. 4d) rapidly rose up to the strongest value with the incubation time progressively increased from 1 h to 12 h (see Fig. 4D–f). Contact regions between Ag NPs on a SERS substrate are always considered to be where SERS-active hot spots generate. A narrow gap between Ag NPs is crucial to the SERS enhancement that can even be tuned by more than 4 orders of magnitude through change in geometry of the Ag NP aggregate [18,41]. The drying steps in the two environments following the same incubation step are obviously involved in different swelling processes of the UF MSs, as shown in Table S1 (Supplementary material). Herein, R6G competitive adsorption between the UF MS (as shown in Fig. S6) and Ag NPs, its migration with limited diffusion, and the swelling of the UF MSs related with change of the gap size between Ag NPs are all considered together to discuss these complex SERS phenomena shown in Fig. 5. R6G molecules during incubation sequentially undergo a limited diffusion process at a low concentration of 1.0 10 7 M and a competitive adsorption one between Ag NPs and FU MS, and then gradually approach to an equilibrium state with the incubation time increasing. The R6G uptake on the Ag NP and UF MS is thereby enough in the long incubation step (see step (a)), but insufficient in the short one (see step (b)). Following the long incubation step (see step (a)) and a subsequent drying one in lab environment, the AgNP/UFMS swollen by 10% (as shown in Table S1b) gave an intense SERS signal from R6G (see spectrum (ad) or Fig. 4c). If dried in a dessicator, however, the sample gave the strongest SERS signal (see spectrum (ac) or Fig. 4f), of which a little of deswelling was found by 2% (as shown in Table S1c). A crucial parameter in a drying method is the water–vapor pressure in the closed space. Dessicator provides an almost absolutely dried environment, but obviously the water partial pressure within a lab environment cannot be ignored. Drying in a dessicator can be considered so a fast and effective process that the size of UF MS shrinks with contraction of capillary pores (see step (c) and Table S1c). The swelling/deswelling of AgNP/UFMS leads to a slight adjustment of gap size between Ag NPs locating at the UF MS and is obviously responsible for the related SERS performance, as also found in literature [33–36]. On the other hand, if the sample is incubated for a short time (see step (b)), two variable factors are prominent for the SERS
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Fig. 4. SERS spectra measured on AgNP/U1F5 dried in a lab environment with 60% humidity (A) or in a dessicator (B) for 12 h following the incubation step in a 1.0 10 7 M R6G solution for 1 h (a, d), 6 h (b, e) or 12 h (c, f).
performance of substrate. One is competitive adsorption of R6G on the UF MS with on the Ag NPs and other one is its subsequent migration or diffusion from the UF MS to the hot spots of Ag NPs, where R6G is more stable under its complexation with Ag NPs and allowed a strong electromagnetic field coupling with itself. Drying steps with different efficiencies can destroy or maintain a hydrophilic environment for R6G to diffuse from the UF MSs to the Ag NPs. When the hydrophilic environment was fast removed in a dessicator (see step (e)), a faint SERS signal was observed (see spectrum (be)), but an intense one (see spectrum (bf)) was found if the sample was dried slowly in a lab environment (see step (f)). Interestingly, the “lost” SERS intensity could be recovered (see spectrum (beg)) if the incubated sample was further treated
within a water–vapor container equilibrated with liquid water at room temperature (see step (g)). One superiority in SERS analysis is its low detection limit, however, it is just this low concentration that may involve analyte in its limited diffusion process [42], especially for a hydrophilic organic analyte on a hydrophilic organic carrier. Some details on the hydrophilicity of UF MSs are illustrated in Table S1 (Supplementary material) as well as our previous works [38]. 3.5. Modification of AgNP/UFMS by inorganic salts Chloride has a strong influence on the formation of Ag NPs [43] and was verified to be an active modifier for improving SERS
Fig. 5. A two-step schematic for R6G incubation followed by drying process of substrate with the SERS performance. AgNP/U1F5 was firstly incubated in a 1.0 10 7 M R6G solution for 12 h (a) or 1 h (b), and then dried in a dessicator (Des) (c or e) or in a lab environment (d or f) for 12 h. A saturated steam-treatment step (g) was designed to recover the SERS performance obstructed by R6G-diffusion limited within the dessicator (e). Swelling and deswelling of UF MS, visualized by size of the UF MS, exist in the long incubation step (a) and the subsequent drying step in the dessicator (c), respectively, and the SERS spectrum after each process is next to the each end.
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Fig. 6. SERS spectra measured on AgNP/U1F5 treated with various 1.0 10 3 M salt solution. (a) AgNO3; (b) KI; (c) Na3C6H5O7; (d) NaAc; (e) Na2SO3; (f) KSCN; (g) KCl; (h) pure H2O; (i) NaHSO4; (j) NaH2PO4.
performance [44]. It is an important step that the sample was treated with a 1.0 10 3 M KCl solution after Ag NPs loaded on. Indeed, anion affinity for Ag þ on Ag NP has been suggested as a competition model [45] and can be used to our SERS practice. Fig. 6 displays R6G SERS spectra measured on AgNP/UFMS treated with a set of different salt solutions. The sample treated with KCl, KSCN or NaHSO4 solution provided a prominent SERS performance. Although most of these salts can react with silver ions so as to produce silver salt precipitates (see Table S1), not all of them can effectively improve the SERS performance. KI, as a typical example, can react with AgNO3 and produce AgI precipitate with a much lower Ksp than that of AgCl or AgSCN, but the substrate treated with it even presented a poorer SERS enhancement (see Fig. 6b) than that treated with AgNO3, Na3C6H5O7 or NaAc solution (see Fig. 6a, c, and d) that hardly react with silver ions (as shown in Table S1). An important parameter in this treating process is the valence of anion in the salt and the SERS substrates treated with salt solutions having a high anion valence (i.e., Na2SO3, NaHSO4 and NaH2PO4) all gave strong backgrounds. The SERS background response to a high anion valence implies that coulomb interaction may exist among the Ag NPs or/and the UF MSs after the treatment. Raman spectrum of UF MSs contains a –CH2 stretching vibration band near 3000 cm 1 and a set of its deformation bands around 1400 cm 1[38] that transforms into a widened background band after its drying process (see Fig. S6a). This background band slightly shifted after the UF MSs treated with a KCl or KSCN solution (see Fig. S6b and c), but quenches and was replaced by a set of weak SERS peaks originating from citrate if it was
treated with citrate-reduced Ag NP solution, as shown in Fig. S6d. Meanwhile, a strong or overflowing background band was observed when the UF MSs was treated with 1.0 10 3 M sodium citrate or 1.0 10 7 M R6G solution, indicating that adsorption of citrate or R6G on the UF MSs occurred after the treating process. Ag NPs from citrate-reduction method can be considered to be stable under protection of the sufficient citrate anions. Herein, the replacement mechanism based on affinity of inorganic anions for Ag NPs can be used in modification of the SERS substrate, of which some details are shown in Fig. S6 (Supplementary material). 3.6. R6G detection limit on AgNP/UFMS SERS spectra measured on AgNP/UFMS incubated with a set of R6G concentrations are shown in Fig. 7. With the concentration decreased from 1.0 10 9 M to 1.0 10 17 M, the SERS signal from R6G gradually declined to a R6G detection limit of 1.0 10 13 M. Therefore, AgNP/UFMS can be used as a simple and effective SERS substrate.
4. Conclusions Dispersing Ag nanoparticles on hydrophilic UF microspheres suggested in this study is a simple and effective method in fabrication of SERS substrate. It is certain that the excess formaldehyde taken in the synthetic reaction of UF microspheres plays an important role in the construction of SERS hot spots, similar to that
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References
Rhodamine 6G
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Raman shift [cm-1] Fig. 7. SERS spectra measured on AgNP/U1F5 incubated in solutions with a set of R6G concentrations, equal to 1.0 10 9 (a), 1.0 10 11 (b), 1.0 10 13 (c), 1.0 10 15 (d) and 1.0 10 17 M (e), respectively. Herein, the spectra were collected at 1% laser power.
illustrated in our last paper [38]. Drying method for either the UF microspheres and the incubated Ag-nanoparticles/UF-microspheres are worthy of careful choices and some interesting details are implicated in the hydrophilicity of UF microspheres. Ag nanoparticle colloidal solution can be prepared by many methods, but only a few of them are frequently employed in SERS detection with some obvious tendency, determined by some inevitabilities from the detection platform itself rather than researcher's preference. The citrate-reduction method employed for Ag nanoparticle colloid in this study is a practical one and a few of its tendency features on Ag-nanoparticles/UF-microspheres were revealed in this study.
Acknowledgments We gratefully acknowledge financial support from the Major Science and Technology Special Project from China State Scientific and Technological Commission (2012YQ1502130202) and from Natural Science Foundation of China (51208367). We are grateful to Shanghai Science and Technology Commission (14DZ2261100).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.talanta.2015.09. 024.
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Ag-nanoparticles on UF-microsphere as an ultrasensitive SERS substrate with unique features for rhodamine 6G detection Zhixian Hao n, Mulati Mansuer, Yuqing Guo, Zhirong Zhu, Xiaogang Wang Shanghai Key Laboratory of Chemical Assessment and Sustainability, Department of Chemistry, Tongji University, Shanghai 200092, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 July 2015 Received in revised form 7 September 2015 Accepted 10 September 2015 Available online 12 September 2015
Urea and formaldehyde (UF) microsphere (MS) adsorbing Ag nanoparticles (NPs) was employed as a surface enhanced Raman scattering (SERS) substrate for rhodamine 6G (R6G) detection. The UF MSs and citrate-reduced Ag colloid supplying Ag NPs are synthesized separately and all the subsequent fabrication procedure is then implemented within 2 mL centrifuge tube. Influences of the composition and drying temperature of the UF MSs and the drying method and modification of AgNP/UFMS on the final SERS performance have first been reported. Excess formaldehyde useful in the formation of UF MSs again plays an important role in the SERS detection. Some interesting phenomena in the approach, such as swelling/deswelling of UF MSs and R6G diffusion within hydrophilic environment of UF MSs, are found to be of variable factors affecting the SERS performance. The substrate AgNP/UFMS confidently achieves a detection limit of 10 13 M R6G and can be used as a simple and effective platform in the SERS spectroscopy. & 2015 Elsevier B.V. All rights reserved.
Keywords: UF microsphere SERS Ag nanoparticles Swelling Hydrophilic
1. Introduction SERS spectroscopy [1,2] has gone through an impressive resurgence in the last three decades, of which the molecular fingerprint specificity with potential single-molecule sensitivity [3] is extremely attractive to chemical and biological detections [4]. When adsorbed on rough metal surfaces or on aggregates of metal NPs, some of analytes under laser excitation have been found to give rise to enhanced Raman signals even by a million-fold or more [5]. Therefore, building metal nanostructures with SERS-active “hot spots” [1,2] has become a significant procedure for SERS detection. Various fantastic Ag/Au nanostructures, such as nanoflowers [6–8], nano-rods [9–11], nano-sheets [12,13], and nanoarrays [14–17], are incredibly fabricated by means of some bottomup assemblies or top-down designs. However, it is difficult to accept these approaches in routine detection because of their complicated fabrication procedures, their high cost, or some inherent defects in their storage. A key practice in SERS detection is to draw adjacent metal NPs at a sufficiently close distance so as to gain SERS-active “hot spots” [18,19] and herein some organic crosslinkers always act as a controlling role. For example, two amino groups in hexamethylene Abbreviations: UF, urea and formaldehyde; MS, microsphere; NP, nanoparticle; SERS, surface enhanced Raman scattering; R6G, rhodamine 6G; AgNP, Ag nanoparticle; UFMS, urea and formaldehyde microsphere n Corresponding author. E-mail address: [email protected] (Z. Hao). http://dx.doi.org/10.1016/j.talanta.2015.09.024 0039-9140/& 2015 Elsevier B.V. All rights reserved.
diamine [20] are responsible for bonding Ag NPs and the hexane chain to hold them at the distance itself. Supporting Ag NPs on functionalized surfaces of some silicon/silica materials is a general accepted method in fabrication of SERS substrate [21–23] and it was confirmed that hydrophilic groups grafted on the surface, such as –NH2 and –SH, can bound metal NPs, but hydrophobic ones, such as –CH3 and –PPh2, cannot [21]. The polymer with more than two short branched chains attaching metal affinity groups at each end is a class of significant crosslinkers in fabrication of SERS substrate. These branched chains working as multi-arms with hydrophilic anchors make assembly of some novel NP architectures much more flexible [24–27]. A recent study [28] verified that the metal NPs can be assembled into a 1-D line, a 2-D plane or a 3-D sphere by mediating the number of chains in a hyperbranched molecule and these NP architectures present a gradually increasing SERS response with the dimensions of themselves increasing, similar to the performance of a gradually enlarged NP cluster [29]. A further work introduces metal NP aggregates into solid/gel polymers or biopolymers with hydrophilic groups by various bottom-up approaches. A uniform and dense Au nano-rod coat on polycaprolactone fibers was achieved through electrospinning technique combining with traditional polyelectrolyte layer-bylayer deposition [30]. Another work grafted fine 3-D Ag nanosheets on polyamide fibers based on electrospinning technique herein uniting with cultivation of Ag nano-sheets from Au NP seeds born in the fibers in situ [31]. Composition of Ag NP spheres within “high-aspect-ratio” benzene tetracarboxylic acid-doped
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polyaniline fiber was recently reported based on a simple solutiondipping method with the fiber itself as both the reducing agent and the carrier [32], which achieves a R6G detection limit equal to 10 10 M. An extreme approach traps metal NPs into poly(N-isopropylacrylamide) microgel [33,34], polyacrylamide particles [35], or agarose gel [36] while swelling/deswelling of the polymer is designed to control the gap size between the metal NPs. Although dense hot spots can be created with numerous hydrophilic groups anchoring at metal NPs from different 3-D points, both an additional swelling/deswelling step and a bottom-up assembly of metal NPs makes these approaches complicated in routine analysis. In previous papers [37,38] we had reported the precipitation reaction of UF MSs and the role of excess monomer in growth of UF deposits. In this one the UF MS adsorbing Ag NPs based on an impregnation method is employed as a SERS substrate. The UF MSs and Ag NP colloidal solution could be acquired and stored separately in a manner of large quantity. The fabrication, incubation and separation of AgNP/UFMS can be simply carried out in mL scale under routine laboratory conditions, while some variable factors in this approach were first described with some appropriate discussion. R6G chosen in this paper is a common biological dye, known to be able to interact with Ag NPs. It is usually employed as a SERS reporter with a larger Raman cross-section in bioassays or for evaluation of SERS substrates.
2. Experimental 2.1. Preparations of UF MSs and Ag NP solution Synthesis of UF MSs was reported previously [37,38]. In simple terms, 2.25 g urea, variable volume of formaldehyde solution (38 wt%) and 40.0 mL deionized water reacted at 28.0 °C for 30 min under catalysis of 0.200 mL hydrochloric acid (37 wt%). The UF MSs was sequentially separated in a Buchner funnel, washed with water and ethanol two times, respectively, dried for 12 h in ambient environment, and then stored in a dessicator for use after dried in oven for 3 h. Herein, the drying temperature of UF MSs and the molar ratio between formaldehyde and urea used in the synthesis reaction were designed as variable factors to investigate the final SERS performance. Ag NP colloidal solution was prepared following a standard procedure [39] with a little of modification. After 5 mL Na3C6H5O7 solution added into 95 mL boiling AgNO3 solution, the beaker was sealed with a piece of plastic paper and held boiling under electromagnetic stirring for 30 min. The initial concentrations of Na3C6H5O7 and AgNO3 added were equal to 3.0 10 3 and 2.0 10 3 M, respectively. The water used in preparations of the Ag NP colloidal solution and the subsequent SERS substrate had an electrical conductivity of 1.37 μS/cm. All chemicals were of analytical grade and used without further purification. 2.2. Fabrication, treatment and incubation of the SERS substrate Before use, Ag NPs within a 2 mL colloidal solution spanning a range of centrifugal rate from 2000 to 6000 rpm was collected and re-dispersed in water to the initial volume, where both the precipitate at the 2000 rpm for 5 min and supernatant at the 6000 rpm for the same period were discarded. Meanwhile, the spanning range of centrifugal rate was changed to investigate its influence on the final SERS performance. Next, 50 mg UF MSs was added into the graded Ag NP solution and separated at 6000 rpm after 60 min impregnation. The AgNP/UFMS was then treated with a 1.0 10 3 M KCl solution or some other inorganic salt solutions to investigate their effect on the SERS performance. Incubation of the AgNP/UFMS after separation was carried out in situ within
2 mL 1.0 10 7 M R6G for 1 h and separated at 6000 rpm following each step. Herein, both the period of incubation and concentrations of R6G were arranged as variable factors to investigate their effect on the SERS performance. 2.3. SERS detection and characteristics of the SERS substrate A little of the incubated AgNP/UFMS was transferred onto a glass slide and dried at ambient temperature in a lab environment with a relative humidity of 45–65% or in a dessicator for 12 h for Raman spectroscopy interrogation. Before the transfer the separated AgNP/UFMS sample in the tube was re-dispersed in 0.3 mL ethanol while the glass slide was treated in ethanol for 10 min under ultrasonic vibration and then a drop of the mixture was used. In order to recover the SERS performance obstructed in a dessicator drying process, the sample was treated for 12 h within saturated water vapor in a dessicator, equilibrated with some liquid water at bottom instead of the silica gel. Raman spectrum was measured on a confocal microscope Raman spectrometer (Invia Reflex Renishaw) with a 514 nm laser as excitation. The laser beam with a spot diameter of ∼1 μm was focused on the surface of sample with an accumulation time of 10 s at a laser power of 20 mW using a 50 microscope objective. Meanwhile the laser power was allowed to choose a less value available to the SERS response. Morphology and element analysis of UF MS or SERS substrate were investigated using a scanning electron microscope (SEM) (Quanta FEG 250, FEI Company) operated at 10.0 kV and an energy dispersive X-Ray spectroscopy (EDS) (Genesis APEX 2, EDAX Company). A little of sample dispersed in ethanol was dropped on a coverslip for the followed measurement. In order to investigate the swelling/deswelling of UF MSs after impregnation or drying process, more than 80 SEM images of UF MSs in a target sample was taken for the statistic average diameter.
3. Results and discussion 3.1. UF MSs adsorbing Ag NPs as SERS substrate Fabrication of SERS substrate AgNP/UFMS was implemented within 2 mL centrifuge tubes. 5.0 mg UF MSs was sequentially impregnated in a Ag NP colloidal solution for 1 h, modified in a 10 3 M KCl solution and incubated in a 10 7 M R6G solution for 1 h, while a centrifugation process at a centrifugal rate of 6000 rpm for 5 min followed each of the steps. A little of the sample was then transferred onto a glass slide and dried in a dessicator for SERS measurement or in a lab environment, as shown in Fig. 1. Ag NP colloidal solution used in the above fabrication presented particles mixed with a small quantity of nano-rods (see SEM image Fig. 1c), which had a maximum absorption around 445 nm, as shown in Fig. S1 (Supplementary material), and a white color instead of a deep grey one. The colloidal solution can be optimized using variable rate range in its centrifugation. Influence of the rate range on SERS performance is shown in Fig. S2, where NP aggregates separated at a low centrifugal rate obviously had a high SERS enhancement, due to large Ag NP aggregates obtained at the low centrifugal rate [29]. 3.2. Influence of the excess formaldehyde on SERS performance It has been shown that the lamellar crystallinity and molecular structure of UF MSs strongly depend on some of variable factors in the synthesis itself, such as the acidity, reactant amounts and the U/F molar ratio. The excess formaldehyde, with respect to the urea
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Fig. 1. Schematic for a simple fabrication procedure for SERS substrate. The U1F5 MSs (a) is firstly impregnated in a Ag NP solution, modified in an inorganic salt solution and incubated in a R6G solution, while a centrifugation step follows each of the above ones. The incubated substrate (b) is then transferred onto a glass slide from the centrifuge tube and dried for SERS detection. Therein, the Ag NPs containing a small amount of nano-rods were randomly dispersed on the surface of a U1F5 MS (c) and the SERS band at Raman shift of 1650 cm 1 collected on the AgNP/U1F5 reached an intensity value more than 5 104 cts after it was incubated within a 10 7 M R6G solution.
used [38], plays an important role in the synthesis reaction of UF MSs. Some excess formaldehyde will finally bond at the terminals of polymer molecules, such as existing in the form of linear molecule HOCH2[HNCONHCH2]nOH, or enrich on the surfaces of UF MSs as a plenty of terminal hydroxymethyl groups –CH2OH. Some of further information on excess formaldehyde is shown in Fig. S3. It is just the variability of excess formaldehyde that allows us to tune the average size of UF molecules or UF MSs [38]. In order to obtain a general understanding to the role of excess formaldehyde in the SERS detection, a set of UF MS samples were firstly synthesized with variable excess formaldehyde at a constant quantity of urea, then dried at 100 °C for 3 h and fabricated into substrate samples. Influence of the excess formaldehyde on the SERS performance is displayed in Fig. 2. The SERS response of R6G on sample AgNP/U1F1 was faint (see Fig. 2a), but the SERS signals on other samples gained intensity gradually with the excess formaldehyde increased in the synthesis of the UF MSs.
The AgNP/U1F5 even presented the most intense SERS signal, reaching to 2.3 104 cts at 614 cm 1 more than 10 times the intensity measured on AgNP/U1F1 (see Fig. S4). The fact that an excellent SERS performance was provided by the substrate originating from excess formaldehyde implies that construction of SERS hot spots could be strongly improved by the plentiful –CH2OH groups (or others with a high oxygen content) enriching on the UF MSs. 3.3. Influence of the drying temperature of UFMS on SERS performance Drying temperature of the UF MSs incorporated in AgNP/UFMS is another factor impacting on the final SERS performance. Pure U1F5 MSs just showed a flat Raman background close to the spectral baseline after treated at different temperature (see Fig. 3a–d). However, the SERS signal collected from R6G incubated
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AgNP/UFMS gained intensity gradually with the drying temperature of the incorporated UF MSs increased from ambient temperature to 100 °C (see Fig. 3e–g). This gradually improved SERS enhancement thoroughly collapsed if the incorporated UF MSs was pretreated at 150 °C for 3 h (see Fig. 3h). If carefully distinguishing the enlarged Raman spectrum measured on the UF MSs treated excessively (see Fig. S5d) from the others (see Fig. S5a–c), a weaker background was found. UF MSs is hydrophilic and was found dehydrated in less than 100 °C [40], while a somewhat low crystallinity was observed as it was treated at 100 °C [40]. These changes are accelerated at 150 °C that should be unfavorable to the SERS analysis.
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Raman shift [cm-1] Fig. 3. Raman spectra collected on U1F5 (a–d) and AgNP/U1F5 (e–h). Before use the U1F5 employed was dried at various temperatures, equal to ambient temperature for 12 h (a, e) and to 50 °C (b, f), 100 °C (c, g), and 150 °C (d, h) for 3 h.
There exist even more complex details in the drying process of R6G-incubated AgNP/UFMS, as shown in Fig. 4. Following an incubation step in a 1.0 10 7 M R6G solution for 1 h, 6 h, or 12 h, a lab environment or a dessicator was adopted to implement the drying process of SERS substrate. An intense R6G SERS signal was observed when the substrate had been dried in a lab environment for 12 h following its incubation for 1 h (see Fig. 4a) and then dropped gradually with the incubation time prolonged from 1 h to 12 h (see Fig. 4a–c). If the substrate had been dried in a dessicator, on the contrary, a faint SERS signal collected from the sample incubated for 1 h (see Fig. 4d) rapidly rose up to the strongest value with the incubation time progressively increased from 1 h to 12 h (see Fig. 4D–f). Contact regions between Ag NPs on a SERS substrate are always considered to be where SERS-active hot spots generate. A narrow gap between Ag NPs is crucial to the SERS enhancement that can even be tuned by more than 4 orders of magnitude through change in geometry of the Ag NP aggregate [18,41]. The drying steps in the two environments following the same incubation step are obviously involved in different swelling processes of the UF MSs, as shown in Table S1 (Supplementary material). Herein, R6G competitive adsorption between the UF MS (as shown in Fig. S6) and Ag NPs, its migration with limited diffusion, and the swelling of the UF MSs related with change of the gap size between Ag NPs are all considered together to discuss these complex SERS phenomena shown in Fig. 5. R6G molecules during incubation sequentially undergo a limited diffusion process at a low concentration of 1.0 10 7 M and a competitive adsorption one between Ag NPs and FU MS, and then gradually approach to an equilibrium state with the incubation time increasing. The R6G uptake on the Ag NP and UF MS is thereby enough in the long incubation step (see step (a)), but insufficient in the short one (see step (b)). Following the long incubation step (see step (a)) and a subsequent drying one in lab environment, the AgNP/UFMS swollen by 10% (as shown in Table S1b) gave an intense SERS signal from R6G (see spectrum (ad) or Fig. 4c). If dried in a dessicator, however, the sample gave the strongest SERS signal (see spectrum (ac) or Fig. 4f), of which a little of deswelling was found by 2% (as shown in Table S1c). A crucial parameter in a drying method is the water–vapor pressure in the closed space. Dessicator provides an almost absolutely dried environment, but obviously the water partial pressure within a lab environment cannot be ignored. Drying in a dessicator can be considered so a fast and effective process that the size of UF MS shrinks with contraction of capillary pores (see step (c) and Table S1c). The swelling/deswelling of AgNP/UFMS leads to a slight adjustment of gap size between Ag NPs locating at the UF MS and is obviously responsible for the related SERS performance, as also found in literature [33–36]. On the other hand, if the sample is incubated for a short time (see step (b)), two variable factors are prominent for the SERS
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performance of substrate. One is competitive adsorption of R6G on the UF MS with on the Ag NPs and other one is its subsequent migration or diffusion from the UF MS to the hot spots of Ag NPs, where R6G is more stable under its complexation with Ag NPs and allowed a strong electromagnetic field coupling with itself. Drying steps with different efficiencies can destroy or maintain a hydrophilic environment for R6G to diffuse from the UF MSs to the Ag NPs. When the hydrophilic environment was fast removed in a dessicator (see step (e)), a faint SERS signal was observed (see spectrum (be)), but an intense one (see spectrum (bf)) was found if the sample was dried slowly in a lab environment (see step (f)). Interestingly, the “lost” SERS intensity could be recovered (see spectrum (beg)) if the incubated sample was further treated
within a water–vapor container equilibrated with liquid water at room temperature (see step (g)). One superiority in SERS analysis is its low detection limit, however, it is just this low concentration that may involve analyte in its limited diffusion process [42], especially for a hydrophilic organic analyte on a hydrophilic organic carrier. Some details on the hydrophilicity of UF MSs are illustrated in Table S1 (Supplementary material) as well as our previous works [38]. 3.5. Modification of AgNP/UFMS by inorganic salts Chloride has a strong influence on the formation of Ag NPs [43] and was verified to be an active modifier for improving SERS
Fig. 5. A two-step schematic for R6G incubation followed by drying process of substrate with the SERS performance. AgNP/U1F5 was firstly incubated in a 1.0 10 7 M R6G solution for 12 h (a) or 1 h (b), and then dried in a dessicator (Des) (c or e) or in a lab environment (d or f) for 12 h. A saturated steam-treatment step (g) was designed to recover the SERS performance obstructed by R6G-diffusion limited within the dessicator (e). Swelling and deswelling of UF MS, visualized by size of the UF MS, exist in the long incubation step (a) and the subsequent drying step in the dessicator (c), respectively, and the SERS spectrum after each process is next to the each end.
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Raman shift [cm-1]
Fig. 6. SERS spectra measured on AgNP/U1F5 treated with various 1.0 10 3 M salt solution. (a) AgNO3; (b) KI; (c) Na3C6H5O7; (d) NaAc; (e) Na2SO3; (f) KSCN; (g) KCl; (h) pure H2O; (i) NaHSO4; (j) NaH2PO4.
performance [44]. It is an important step that the sample was treated with a 1.0 10 3 M KCl solution after Ag NPs loaded on. Indeed, anion affinity for Ag þ on Ag NP has been suggested as a competition model [45] and can be used to our SERS practice. Fig. 6 displays R6G SERS spectra measured on AgNP/UFMS treated with a set of different salt solutions. The sample treated with KCl, KSCN or NaHSO4 solution provided a prominent SERS performance. Although most of these salts can react with silver ions so as to produce silver salt precipitates (see Table S1), not all of them can effectively improve the SERS performance. KI, as a typical example, can react with AgNO3 and produce AgI precipitate with a much lower Ksp than that of AgCl or AgSCN, but the substrate treated with it even presented a poorer SERS enhancement (see Fig. 6b) than that treated with AgNO3, Na3C6H5O7 or NaAc solution (see Fig. 6a, c, and d) that hardly react with silver ions (as shown in Table S1). An important parameter in this treating process is the valence of anion in the salt and the SERS substrates treated with salt solutions having a high anion valence (i.e., Na2SO3, NaHSO4 and NaH2PO4) all gave strong backgrounds. The SERS background response to a high anion valence implies that coulomb interaction may exist among the Ag NPs or/and the UF MSs after the treatment. Raman spectrum of UF MSs contains a –CH2 stretching vibration band near 3000 cm 1 and a set of its deformation bands around 1400 cm 1[38] that transforms into a widened background band after its drying process (see Fig. S6a). This background band slightly shifted after the UF MSs treated with a KCl or KSCN solution (see Fig. S6b and c), but quenches and was replaced by a set of weak SERS peaks originating from citrate if it was
treated with citrate-reduced Ag NP solution, as shown in Fig. S6d. Meanwhile, a strong or overflowing background band was observed when the UF MSs was treated with 1.0 10 3 M sodium citrate or 1.0 10 7 M R6G solution, indicating that adsorption of citrate or R6G on the UF MSs occurred after the treating process. Ag NPs from citrate-reduction method can be considered to be stable under protection of the sufficient citrate anions. Herein, the replacement mechanism based on affinity of inorganic anions for Ag NPs can be used in modification of the SERS substrate, of which some details are shown in Fig. S6 (Supplementary material). 3.6. R6G detection limit on AgNP/UFMS SERS spectra measured on AgNP/UFMS incubated with a set of R6G concentrations are shown in Fig. 7. With the concentration decreased from 1.0 10 9 M to 1.0 10 17 M, the SERS signal from R6G gradually declined to a R6G detection limit of 1.0 10 13 M. Therefore, AgNP/UFMS can be used as a simple and effective SERS substrate.
4. Conclusions Dispersing Ag nanoparticles on hydrophilic UF microspheres suggested in this study is a simple and effective method in fabrication of SERS substrate. It is certain that the excess formaldehyde taken in the synthetic reaction of UF microspheres plays an important role in the construction of SERS hot spots, similar to that
Z. Hao et al. / Talanta 146 (2016) 533–539
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References
Rhodamine 6G
Intensity [a.u.]
1000 cts
[cts]
(a)
10-9
-11 (b) 10 -13 (c) 10
(d) 10-15 (e) 10-17 600
800
1000
1200
1400
1600
Raman shift [cm-1] Fig. 7. SERS spectra measured on AgNP/U1F5 incubated in solutions with a set of R6G concentrations, equal to 1.0 10 9 (a), 1.0 10 11 (b), 1.0 10 13 (c), 1.0 10 15 (d) and 1.0 10 17 M (e), respectively. Herein, the spectra were collected at 1% laser power.
illustrated in our last paper [38]. Drying method for either the UF microspheres and the incubated Ag-nanoparticles/UF-microspheres are worthy of careful choices and some interesting details are implicated in the hydrophilicity of UF microspheres. Ag nanoparticle colloidal solution can be prepared by many methods, but only a few of them are frequently employed in SERS detection with some obvious tendency, determined by some inevitabilities from the detection platform itself rather than researcher's preference. The citrate-reduction method employed for Ag nanoparticle colloid in this study is a practical one and a few of its tendency features on Ag-nanoparticles/UF-microspheres were revealed in this study.
Acknowledgments We gratefully acknowledge financial support from the Major Science and Technology Special Project from China State Scientific and Technological Commission (2012YQ1502130202) and from Natural Science Foundation of China (51208367). We are grateful to Shanghai Science and Technology Commission (14DZ2261100).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.talanta.2015.09. 024.
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