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Polyoxovanadate fabricated gold nanoparticles: Application in SERS
Journal of Colloid and Interface Science 487 (2017) 209–216
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Regular Article
Polyoxovanadate fabricated gold nanoparticles: Application in SERS Bharat Baruah ⇑, Toni-Ann Miller Department of Chemistry and Biochemistry, Kennesaw State University, Kennesaw, GA 30144-5591, United States
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 25 September 2016 Accepted 15 October 2016 Available online 17 October 2016 Keywords: Gold nanoparticles Polyoxometalate Surface modification SERS Analyte
a b s t r a c t This article reports a surface modification of gold nanoparticles with water soluble polyoxometalate, V10O6 28 (decavanadate, V10). Two sizes of citrate-capped gold nanoparticles AuNP-Citrate-S (11 nm) and AuNP-Citrate-L (46 nm) were modified with V10 in aqueous media to form AuNP-V10-S and AuNP-V10-L, respectively. Both AuNP-V10-S and AuNP-V10-L were found to be significantly better than their citrate counterparts in strengthening Raman vibrational signals of analyte molecule. All the nanoparticles were characterized by UV–visible and Fourier transform infrared (FTIR) spectroscopies, dynamic light scattering (DLS), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) line analysis. We demonstrate that AuNP-V10-L is excellent surface-enhanced Raman scattering (SERS) substrate for a Raman-active analyte molecule at nanomolar concentrations. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction Nanoparticles are considered intermediate between bulk material and individual atoms and known to exhibit unique sizedependent characteristics [1]. In the current decade, nanoparticles have attracted enormous scientific interest due to the striking physical and chemical properties which can be altered by tweaking their configuration, morphology, size and diverse synthetic proce⇑ Corresponding author. E-mail address: [email protected] (B. Baruah). http://dx.doi.org/10.1016/j.jcis.2016.10.036 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
dures [2]. In addition, notable attention has been dedicated to gold nanoparticles (AuNPs) ascribable to their extensive prospective application in biosensing [3], chemical sensing [4], photonics [5], electrocatalysis [6], cancer therapy [7] and surface-enhanced Raman scattering (SERS) detection [8]. Nanostructured gold is synthesized by using a variety of reducing agents, namely, sodium citrate [9], sodium borohydride [10], ascorbic acid [11], hydrazine [12], etc. In nanoparticles synthesis, utilization of capping agent or ligand is common and are predominantly used to avoid aggregation and ensure monodispersed nanoparticles [13]. Amongst metal nanoparticles gold nanoparticles are straightforward to synthesize
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and relatively stable [14]. However, achieving monodispersed gold nanoparticles in the absence of commonly used citrate [15] or alkylthios [16] is often challenging [14]. Despite the fact various capping agents namely, amines [17], carboxylates [18], porphyrins [19], polymers [20], dendrimers [21], protein [22], and DNA [23] are used as a capping agent to stabilize gold nanoparticles. Recently, polyoxometalate capped nanoparticles have been reported [24–28]. Polyoxometalates (POM) are well defined metal-oxygen cluster anions of early-transition metals, commonly having d0 or d1 electronic configurations (e.g., V(V), Nb(V), Mo(VI) and Mo(V), or W(VI)), bridged by oxygen atoms (formally O2, or occasionally HO, ions) [24,26]. Ernst et al. demonstrated chemisorption of Keggin-type phosphododecamolybdate (PMo12O3 40 , PMo12) on AuNPs surfaces by partial replacement of alkanothiolatemodified AuNPs [29]. First synthesizing AuNPs achieved the PMo12-AuNP in toluene and then performing ligand-place exchange and phase transfer to aqueous solutions [29]. These PMo12-AuNP subsequently immobilized on glassy carbon and further fabricated with a polymer (PANI) [29]. The fabricated glassy carbon electrode containing POM, conducting polymer and AuNPs used for various electrocatalytic applications [29]. Zoladek and co-workers [28] developed and described a unique chemical method of fabrication of phosphomolybdate (PMo12O3 40 , PMo12) stabilized AuNPs. The uniform AuNP with a monolayer of phosphomolybdate used as support for immobilized electrocatalytic platinum particles. The composite performed efficient electrooxidation of ethanol [28]. Cheng et al. [24] demonstrated the formation of PMo12-capped AuNPs by the chemical synthesis. They have further shown that PMo12-capped AuNPs can be immobilized on ITO electrode with the help of PVP. This monolayer and multilayer PMo12-capped AuNPs modified ITO electrodes exhibited an excellent catalytic reduction of iodate [24]. A series of recent work by Wang and co-workers [25–27,30] exhibited in-situ formation of POM capped AuNPs revealing influence of metal cation size on the stability of POM monolayer on gold nanoparticles [25,27]. Furthermore, lability and reversible binding of the POM ligand on gold nanoparticle surface would facilitate undeviating access of the substrate molecules to the gold atoms on the AuNPs surface and thereby ensure optimum catalytic activity [30]. In the present work, we demonstrate two-step synthesis of AuNPs stabilized by a polyoxometalate anion called decavanadate, V10O6 28 (V10) (Fig. 1) which is stable in aqueous solution from pH 3 to 6 [31]. We first synthesized citrate-capped AuNPs, AuNP-Citrate [32,33] and then modified to V10-capped AuNPs, AuNP-V10 by ligand displacement method [34]. Two different sizes of AuNP-Citrate are prepared and further altered to AuNP-V10. We
characterized these nanoparticles by UV–visible and Fourier Transformed Infrared (FTIR) spectroscopies and dynamic light scattering (DLS), transmission electron microscopy (TEM), and energy dispersive (EDX) line analysis techniques. The SERS activity of these nanoparticles assessed with Raman spectroscopy. 2. Materials and methods 2.1. Materials NaAuCl42H2O (99%) and crystal violet (CV) (99%) were purchased from Sigma-Aldrich; trisodium citrate dihydrate (98%), ammonium metavanadate (98%), and hydroquinone (98%) were obtained from Fisher Scientific. All chemicals and solvents were used without further purification. Glassware were rinsed with aqua regia (3:1 v/v HCl (37%)/HNO3 (65%) solutions) and then rinsed thoroughly with DI H2O before use. Caution: aqua regia solutions are dangerous and highly corrosive. Aqua regia should be used with extreme care. Fresh aqua regia solutions should not be stored in closed containers. The DI water in all experiments was Milli-Q water (18 MX cm, Millipore). 2.2. Synthesis of citrate-capped small gold nanoparticles (AuNPCitrate-S) Citrate-capped nanoparticles were synthesized following literature methods [32,33]. 300 lL of 1.0% NaAuCl42H2O was placed in an Erlenmeyer flask that contained 30 mL of DI water. The mixture was brought to boiling rapidly while stirring. When the mixture came to a boil, 900 lL 1.0% sodium citrate was added. A series of color change was observed from blue, dark purple to final wine red. Thus formed AuNP-Citrate-S was purified by centrifuging at 12,000 rpm for 20 min in 1.5 mL batches in 1.5 mL eppendorf tubes. The supernatant was discarded, and the sample was redispersed in 1.5 mL pure DI water. The centrifugation as above was repeated twice, and nanoparticles were stored for further application. 2.3. Preparation of V10 capped small gold nanoparticles (AuNP-V10-S) Small V10-capped nanoparticles were prepared by replacing the citrate form the surface of AuNP-Citrate-S synthesized above. AuNP-Citrate-S was centrifuged for 20 min at 12,000 rpm. The pH of the solution was adjusted to 4.8 by adding 10 mM hydrochloric acid. To the 10 mL of pH adjusted AuNP-Citrate-S solution, 500 lL
Fig. 1. The molecular structure of decavanadate ion, V10O6 28 .
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of 100 mM ammonium metavanadate solutions at pH 3.8 (10 mM V10) was added. The solution was centrifuged at 10,000 rpm for 15 min in 1.0 mL batches in 1.5 mL eppendorf tubes. After that the supernatant was discarded and the sample was redispersed in 1.0 mL pure DI water, and 50 lL of V10 was added to each 1.0 mL batch. The centrifugation as above was repeated twice and final AuNP-V10-S thus formed were stored for further application. 2.4. Synthesis of citrate-capped large gold nanoparticles (AuNPCitrate-L) In the synthesis of large citrate-capped AuNPs, 100 lL of 1% NaAuCl42H2O solution was added to 7420 lL of DI water, in a 20 mL vial. To the 20 mL vial, 378 lL of the above prepared AuNP-Citrate-S solution was added as nanoparticle seed. The mixture was then stirred rapidly at room temperature for five minutes. Then, 22 lL of a 1% sodium citrate solution, immediately followed by 100 lL of 30 mM hydroquinone solution was added. The vial was allowed to stir for 10 min while reduction of gold salt took place to form an extra layer gold on the seed nanoparticles rendering large AuNP-Citrate-L [33]. Thus formed AuNP-Citrate-L was purified by centrifuging at 6000 rpm for 20 min in 1.0 mL batches in 1.5 mL eppendorf tubes. The supernatant was discarded, and the sample was redispersed in 1.0 mL pure DI water. The centrifugation as above was repeated twice, and nanoparticles were stored for further application. 2.5. Preparation of V10 capped large gold nanoparticles (AuNP-V10-L) Large V10-capped nanoparticles were prepared by following the same synthetic procedure as stated above for AuNP-V10-S. The only difference is, AuNP-Citrate-L was used as a precursor. The solution was centrifuged at 4000 rpm for 10 min in 1.0 mL batches in 1.5 mL eppendorf tubes after adding 100 lL of 100 mM ammonium metavanadate solutions at pH 3.8 (10 mM V10) was added to each 1.0 mL batch. After that, the supernatant was discarded, and the sample was redispersed in 1.0 mL pure DI water. The centrifugation as above was repeated twice and final AuNP-V10-L thus formed were stored for further application. 2.6. UV–visible and Fourier transform infrared spectroscopic measurement of nanoparticles The absorption spectrum was recorded using a Cary 4000 UV– visible spectrophotometer. FTIR measurement was performed using Perkin-Elmer FTIR Spectra 100 spectrometer fitted with diamond ATR. 2.7. Raman spectroscopic measurement The CV containing AuNP-Citrate-S, AuNP-Citrate-L, AuNP-V10S, and AuNP-V10-L colloids are well monodispersed in pure DI water. 2.5 lM and 50 lM solutions of CV in methanol are used as stock. Typical samples for SERS contained 1.0 mL of colloids dispersed in pure DI water and an aliquot of 4.0 or 20 lL of 2.5 lM CV (in MeOH) so that the final concentration of CV are 10 or 50 nM; an aliquot 2.0 or 5.0 or 10 lL of 50 lM CV (in MeOH) so that the final concentration of CV are 100 or 250 or 500 nM. Samples are allowed to stand for 1 h before Raman spectroscopic measurements are carried out using a DeltaNu Advantage 200A Raman spectrometer. This instrument is equipped with a HeNe laser set at 632.8 nm. The integration time for all measurements was 10 s. The Raman spectrum of 10 mM CV was acquired with an integration time of 1.0 s. However, the spectral resolution of 10 mM CV was weak as
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its Raman signal sits on top of a strong fluorescence background [35].
2.8. Electron microscopy and EDX analysis Transmission electron microscopy (TEM) morphologies were gauged with Hitachi H-7500 transmission electron microscope at an accelerating voltage of 75 kV, Hitachi H-7600 transmission electron microscope at an accelerating voltage of 120 kV and Hitachi H-9500 HR-TEM microscope with LaB6 source of resolution 0.1 nm at an accelerating voltage of 300 kV. Nanoparticle samples prepared in DI water and were centrifuged and redispersed in DI water. These solutions were filtered with 200 nm syringe filters before applying on grids for TEM measurements. Samples were prepared by spreading a 3.0 lL of the colloidal sample on an ultrathin 300 mesh Formvar/carbon-coated copper grid, dried in air. Energy dispersive X-ray (EDX) line analysis was performed using Scanning Transmission Electron Microscope (STEM) HD2000 with Field Emission source of resolution 0.24 nm at 200 kV.
2.9. Dynamic light scattering (DLS) measurements The mean hydrodynamic diameter of AuNP-Citrate-S, AuNPCitrate-L, AuNP-V10-S, and AuNP-V10-L colloids were determined using a commercial Zetasizer (Malvern Zetasizer Nano ZS, Malvern Instruments). Samples were loaded into disposable cells and measured the particle sizes twice and in triplicate. Zeta potential measured in a disposable cell with a dip probe.
3. Results and discussion It is evident from current research that polyoxometalate has been used by researchers to stabilize gold nanoparticles [24,26,28,29]. POM-AuNP prepared by chemical synthesis [24,28], ligand replacement [29], and in-situ synthesis [25,27,30]. We have modified two different sizes of AuNP-Citrate into the corresponding AuNP-V10 by applying simple ligand displacement method [29] in aqueous solutions. In a typical synthesis colloids of AuNPCitrate treated with an aqueous solution of V10 and further centrifuged to get rid of displaced citrate ions. This process repeated few times to ensure complete displacement of citrate by V10. Scheme 1 demonstrates the synthetic route described above. Scheme 1 also depicts the formation of AuNP-Citrate-L from AuNP-Citrate-S by seed growth method. Both AuNP-V10-L and AuNP-V01-S colloids are better SERS substrates compared to their citrate counterparts AuNP-Citrate-L and AuNP-Citrate-S, respectively.
3.1. Characterization of colloids by UV–visible spectroscopy Fig. 2A represents the UV–visible spectra of all the four different colloids. AuNP-Citrate-S and AuNP-Citrate-L show surface plasmon resonance bands at absorption maximum of 519 and 529 nm, respectively. The AuNP-Citrate-L synthesized from AuNP-CitrateS by seed growth method [33]. Apparently, the red shift of the UV–visible absorption maximum from AuNP-Citrate-S to AuNPCitrate-L support the size increase in nanoparticles. Similarly, the AuNP-V10-S and AuNP-V10-L show SPR bands at absorption maximum of 525 and 533 nm, respectively. Fig. 2A inset exhibits the digital photographs of all four colloid samples.
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Scheme 1. Syntheses of AuNP-Citrate-S, AuNP-Citrate-L, AuNP-V10-S, and AuNP-V10-L colloids.
Fig. 2. (A) UV–visible spectra and (B) dynamic light scattering size distribution of nanoparticle samples: (a) AuNP-citrate-S (black line), (b) AuNP-V10-S (red line), (c) AuNPcitrate-L (blue line), and (d) AuNP-V10-L (pink line). In the inset, we show digital photographs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. Size and zeta potential of nanoparticles with dynamic light scattering Fig. 2B shows particle size distribution (PSD) of AuNP-Citrate-S, AuNP-V10-S, AuNP-Citrate-L, and AuNP-V10-L measured using dynamic light scattering (DLS). AuNP-citrate-S displays a mean hydrodynamic diameter of 11.0 nm (average PDI = 0.220). We prepare AuNP-V10-S colloids by displacement of citrate in AuNPCitrate-S with V10 solution. AuNP-V10-S remain monodispersed with a mean hydrodynamic diameter of 14.0 nm (average PDI = 0.325) (Fig. 2B). We synthesize AuNP-Citrate-L from AuNPCitrate-S by seed growth method [33] and then modify further to AuNP-V10-L following the same method described above. Both AuNP-Citrate-L and AuNP-V10-L persist monodispersed in water with hydrodynamic diameter of 46.0 nm (average PDI = 0.125) and 51.0 nm (average PDI = 0.320), respectively. The sizes of these nanoparticles are also measured from the TEM images using imageJ software, and the measurements are comparable to those obtained from DLS measurements. The size distribution histograms are shown in Supplementary material (Fig. S1). As listed in Table 1 the electrophoretic mobility measurements for the above four colloid samples was done by zeta potential values obtained using commercial Zetasizer. The stability of the dispersed nanoparticles depends on their surface charge and the pH [36]. We measured zeta potential of all four colloid samples at pH 4.8 to verify their stability. Zeta potentials for AuNP-Citrate-S, AuNP-V10-S, AuNP-Citrate-L, and AuNP-V10-L colloids are 32.0 ± 2.0, 36.0 ± 2.0, 33.0 ± 2.0, and 35.0 ± 2.0, respectively.
Table 1 Mean hydrodynamic diameter and zeta potential of colloidal solutions. Colloidal samples
Size (d, nm)
Zeta potential (mV)
AuNP-Citrate-S AuNP-V10-S AuNP-Citrate-L AuNP-V10-L
11.0 ± 1.0 14.0 ± 1.0 46.0 ± 1.0 51.0 ± 1.0
32.0 ± 2.0 36.0 ± 2.0 33.0 ± 2.0 35.0 ± 2.0
The negative zeta potentials indicate negatively charged surface ligand in these colloid samples. 3.3. TEM images and EDX analysis of nanoparticles The morphologies of the nanoparticles were imaged using TEM. The TEM images of nanoparticles AuNP-Citrate-S, AuNP-V10-S, AuNP-Citrate-L, and AuNP-V10-L presented in Fig. 3A–D, respectively. The particle size distributions obtained (Fig. S1) from these images are comparable to those calculated by DLS in solution. The elemental composition of all four nanoparticles determined using EDX measurement with a Hitachi HD-2000 STEM at an operating voltage of 200 kV. In the EDX spectra of all four samples, the signature signal for gold appears at 2.15 keV (Fig. S2) [37]. For sample AuNP-V10-S (Fig. S2B) and AuNP-V10-L (Fig. S2D), one additional peak appears near 5.0 keV indicating the presence of V atom. This signal corresponds to previously reported EDX value for V atom [38]. Additional background signals near 0.13 and 8 keV are due to C and Cu present in Formvar/carbon and coated copper grid, respectively.
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Fig. 3. TEM images of (A) AuNP-Citrate-S (scale bar 100 nm), (B) AuNP-V10-S (scale bar 50 nm), (C) AuNP-Citrate-L (scale bar 100 nm), and (D) AuNP-V10-L (scale bar 50 nm) samples. Images were taken with a Hitachi H-7500 transmission electron microscope at an accelerating voltage of 75 kV (image A), Hitachi H-7600 transmission electron microscope at an accelerating voltage of 120 kV (image C) and Hitachi H-9500 HR-TEM Microscope at an accelerating voltage of 300 kV (images B and D). Aqueous sample stretch out on an ultrathin 300 mesh Formvar/carbon film on a copper grid and dried in air.
3.4. Characterization of colloids by FTIR spectroscopy We present the FTIR spectra in Fig. 4. Spectrum (a) depicts stretching frequencies for sodium citrate [39] at 3449/3244 cm1,
2966/2925 cm1, 1581 cm1, 1387 cm1, 1080/949 cm1 and 752 cm1 are attributed to free/H-bonded OAH, CAH stretch, C@O stretch, OAH in-plan bending, CAO stretch, and out-of-plan OAH bending. Spectrum (b) shows the stretching frequencies for V10 [40] at 3162/3033 cm1, 1405 cm1, 956 cm1, 831 cm1, and 744 cm1 attributed to OAH stretching, OAH bending, V@O stretch, asymmetric stretching of bridging VAOAV, and symmetric stretching of bridging VAOAV. As our nanoparticle samples and V10 stock solutions are acidic pHs (pH 4–5), we expect to have 3 H2V10O4 28 and H3V10O28 species in AuNP-V10-S and AuNP-V10-L [31]. For the nanoparticles, the samples were first dried on the ATR slot and then FTIR data were collected. For AuNP-Citrate-L and AuNP-Citrate-S we observe 2922/2857 cm1, 1563 cm1, and 1082 cm1 attributed to the CAH stretch, C@O stretch, and CAO stretch, respectively. The presence of these signals indicate that citrate is on the AuNPs surface. On the other hand for AuNP-V10L and AuNP-V10-S samples we observe 3190/3061 cm1, 1424 cm1, 962 cm1, 842 cm1, and 739 cm1 attributed to the OAH stretch, OAH bending, V@O stretch, asymmetric VAOAV stretching, and symmetric VAOAV stretching, respectively. The presence of characteristic V@O and VAOAV vibrational signals in both AuNP-V10-L and AuNP-V10-S colloids indicate that the nanoparticles surface has capped with V10. 3.5. AuNP-citrate and AuNP-V10 as SERS substrates
Fig. 4. FTIR spectra of (a) solid trisodium citrate (black line), (b) V10O6 28 (V10) (red line), (c) AuNP-Citrate-L (blue line), (d) AuNP-V10-L (orange line), (e) AuNP-CitrateS (cyan line), and (f) AuNP-V10-S (pink line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Dispersed gold nanoparticles can carry [41] the surface plasmon resonance band even in the presence of a diverse organic analyte molecule, and hence these are satisfactory SERS substrates. Very recently, Lai et al. fabricated [42] mesostructured polyoxometalate(POM)-silica surfactant (PSS) template containing 3D densely spaced AuNPs of 2 nm [42]. This new composite material display notable SERS enhancement with adsorbed 4-mercaptobenzoic acid due to the presence of 3D nanometer-spaced AuNPs [42]. Another
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study reported the size- and shape-controlled synthesis of complex silver dendrites based on galvanic displacement reaction assisted by the presence of a POM [43]. The POM both served as reductant and stabilizer and these POM based silver dendrite exhibited strong SERS signal for Rhodamine B [43]. The AuNP-Citrate-L, AuNP-Citrate-S, AuNP-V10-L, and AuNP-V10-S nanoparticles, synthesized in the current work were assessed for SERS using crystal violet (CV), a Raman-active analyte. We observed a dramatic difference in SERS activity among the four colloids mentioned above. The SERS activity of the above four colloids is examined using CV as the Raman-active analyte. Fig. 5A shows SERS spectra AuNP-Citrate-L samples in the presence of CV by varying the concentration from 0.0 nM to 500 nM using a laser excitation wavelength of 633 nm. The SERS signal of CV is noticeable only in the sample of AuNP-Citrate-L with 500 nM CV (Fig. 5A (f), pink line). All of these samples were prepared by adding calculated amount of CV stock solutions in MeOH to AuNP-Citrate-L. After that, the AuNP-Citrate-L samples are incubated for 1 h before SERS measurements. The SERS signals of CV are allocated [44] as strong ring CAC stretching at 1621 cm1, N-phenyl stretching at 1383 cm1, ring CAH bending at 1173 cm1, medium (CCcenterC) at 916 cm1, and medium signal at 803 cm1. The SERS signals are slightly shifted from the Raman vibrational modes at 1619, 1378, 1169, 913, and 796 cm1 reported in the literature [44,45]. Fig. 5B shows the corresponding of surface plasmon resonance bands of all of the above colloids of AuNP-Citrate-L in presence and absence of CV. Sample (f) (the pink line) has an extra peak at 725 nm attributed to aggregation of AuNP-Citrate-L in the presence of 500 nM CV. Fig. 5B inset depicts digital photographs of all these samples with and without CV. Fig. 6A shows the SERS signal of CV with AuNP-V10-L in the presence of CV by varying the concentration from 0.0 nM to 500 nM. The SERS signal of CV is noticeable for the sample of AuNP-V10-L with 50 nM to 500 nM CV (Fig. 6A (c)–(f)). All of these samples were prepared by adding calculated amount of CV stock solutions in MeOH to AuNP-V10-L. After addition of CV aliquot, the AuNP-V10-L samples are incubated for 1 h before SERS measurements. The SERS signals of CV are allocated [44] as strong ring CAC stretching at 1620 cm1, N-phenyl stretching at 1374 cm1, ring CAH bending at 1174 cm1, medium (CCcenterC) at 917 cm1, and medium signal at 805 cm1. The SERS signals are slightly shifted from the Raman vibrations at 1619, 1378, 1169, 913, and 796 cm1 reported in the literature [44,45]. Fig. 6B represents the plot of SERS intensity versus concentration of CV for the most intense Raman active signals namely
1174, 1374, and 1620 cm1. As the concentration increases the SERS intensity increases. Raman mode at 1620 cm1 and 1374 cm1 have equal intensities and the mode at 1174 cm1 has the lowest intensity for 500 nM CV. For the other samples with 50 nM CV to 250 nM CV, all three have comparable intensities. Fig. 6C shows surface plasmon resonance bands of all of the above colloids of AuNP-V10-L in presence and absence of CV. There is no sign of nanoparticle aggregation even in the presence of 500 nM CV. Fig. 6C inset shows digital photographs of all these samples with and without CV. Fig. 7A shows the SERS signal for AuNP-V10-S and AuNPCitrate-S in the presence of CV at 500 nM. It is clear that in the presence of AuNP-V10-S we observe the typical SERS modes of CV at 1180, 1377, and 1620 cm1. However, in the presence of AuNP-Citrate-S, we do not observe any of those signals. Both samples were prepared as explained above. In Fig. 7B we present the UV–visible spectra of AuNP-Citrate-S and AuNP-V10-S. To evaluate SERS performance, we determine the analytical enhancement factor (AEF) [46]. AEF of a nanoparticles sample is determined from the ratio of SERS intensity (ISERS) of a vibrational mode in question of the selected analyte and the corresponding Raman intensity (IRS) under equivalent experimental settings (e.g. laser power, integration time, sample preparation, etc.) using the equation [46]:
AEF ¼
ISERS C RS IRS C SERS
ð1Þ
CRS and CSERS are the concentrations of the analyte in the Raman and SERS experiments, respectively. For spectrum (f) of Fig. 5A, CSERS = 5.0 10–7 M, and ISERS (1621 cm–1) = 5618. The corresponding CRS = 0.01 M and IRS (1621 cm–1) = 1489. The AEF was calculated to be 7.6 104. For spectrum (f) of Fig. 6A, CSERS = 5.0 10–7 M, and ISERS (1620 cm1) = 19,204 and the AEF was calculated to be 2.6 105. This value is close to the highest AEF documented in the literature for metal-based nanoparticles [46]. Similarly, for spectrum (b) of Fig. 7A, CSERS = 5.0 10–7 M, and ISERS (1620 cm–1) = 2212 and the AEF was calculated to be 3.0 104. In Table 2 we summarize the AEF for all the samples for the most intense Raman shift near 1174, 1377, and 1620 cm1. All the nanoparticle samples have a negative charge at the surface, and the analyte molecule CV has a positive change. Electrostatically, they would attract each other. As indicated in Table 2 for AuNP-V10-L colloids, the Raman mode at 1620 cm1 has the highest AEF of 2.6 105 and Raman mode at 1374 cm1 has minimum AEF of 1.4 105. AuNP-Citrate-L samples follow the same trend.
Fig. 5. (A) SERS spectra of AuNP-Citrate-L in presence of (a) 0.0 nM CV (black line), (b) 10 nM CV (red line), (c) 50 nM CV (blue line), (d) 100 nM CV (orange line), (e) 250 nM CV (cyan line), and (f) 500 nM CV (pink line); (B) UV–visible spectra are shown for all the above AuNP-Citrate-L samples. Inset shows the corresponding digital photographs of these colloids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. (A) SERS spectra of AuNP-V10-L in presence of (a) 0.0 nM CV (black line), (b) 10 nM CV (red line), (c) 50 nM CV (blue line), (d) 100 nM CV (orange line), (e) 250 nM CV (cyan line), and (f) 500 nM CV (pink line); (B) The corresponding plots of Raman intensity versus concentrations are for the most intense Raman active modes (1620, 1374 and 1174 cm1) of CV; (C) UV–visible spectra are shown for all the above AuNP-V10-L samples. Inset shows the corresponding digital photographs of these colloids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. (A) SERS spectra of (a) AuNP-Citrate-S in the presence of 500 nM CV (black line) and (b) AuNP-V10-S in the presence of 500 nM CV (red line); (B) UV–visible spectra are shown for both colloids solutions. Corresponding digital photographs are in the inset. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 2 List of Raman shift, SERS intensity, Raman dye concentration and analytical enhancement factor (AEF) of CV in the presence of AuNP-Citrate-L, AuNP-V10-S, and AuNP-V10-L. Sample
Raman shift (cm1)
SERS intensity
CV (nM)
AEF
AuNP-Citrate-L AuNP-V10-S AuNP-V10-L AuNP-Citrate-L AuNP-V10-S AuNP-V10-L AuNP-Citrate-L AuNP-V10-S AuNP-V10-L
1173 1180 1174 1382 1377 1374 1621 1620 1620
6720 4051 17,894 6439 4191 20,076 5618 2212 19,204
500 500 500 500 500 500 500 500 500
6.4 104 5.4 104 1.6 105 5.2 104 5.6 104 1.4 105 7.6 104 3.0 104 2.6 105
The colloidal systems AuNP-V10-L has enhancement factor in order of 105 and AuNP-Citrate-L has enhancement factors in order of 104. Even AuNP-V10-S has enhancement factor in order of 104. 4. Conclusion In conclusion, we report the fabrication of AuNP-V10-L and AuNP-V10-S by simple ligand displacement reaction of AuNPCitrate-L and AuNP-Citrate-S, respectively. These colloids are highly stable in aqueous solution and can be stable in the presence of analyte molecules. AuNP-V10-L is 5 nm larger, and AuNP-V10S is 3 nm greater than their corresponding precursors. Red-shift in UV–visible spectra, dynamic light scattering size distribution
and TEM images support this size growth. Red shift in SPR band and FTIR data support surface modification of AuNP by V10. EDX line analysis (Supporting information) reinforce the presence of V-atom in AuNP-V10-S and AuNP-V10-L colloids. In this work we have also shown that AuNP-Citrate-L and AuNP-V10-L nanoparticles augment the Raman signal of adsorbed analyte molecule. The analytical enhancement factor of SERS for AuNP-Citrate-L and AuNP-V10-L are in order of 104 and 105, respectively. AuNPCitrate-L yield SERS signal at 500 nM analyte concentration, whereas AuNP-V10-L enables SERS signal of the analyte at as low as 50 nM. On the other hand, AuNP-V10-S enable SERS signal at 500 nM analyte concentration, but AuNP-Citrate-S do not yield any signal at that concentration. Further research is in progress.
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Acknowledgements The authors thank the Department of Chemistry and Biochemistry, CSM Mentor Protégé fund (BARUAH20FY2013-01) and OVPR fund at Kennesaw State University. Support from Drs. Mark Anderson, College Dean and Mark B. Mitchell, Department Chair are greatly appreciated. Advanced Materials Research Laboratories (AMRL) at Clemson University for TEM and SEM and Dr. David Gottfried at IEN, Georgia Institute of Technology for dynamic light scattering studies are acknowledged. 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.jcis.2016.10.036. References [1] G. Schmid, Nanoparticles: From Theory to Application, Wiley, 2006. [2] G. Schmid, B. Corain, Nanoparticulated gold: syntheses, structures, electronics, and reactivities, Eur. J. Inorg. Chem. (2003) 3081–3098. [3] X. Cao, Y. Ye, S. Liu, Gold nanoparticle-based signal amplification for biosensing, Anal. Biochem. 417 (2011) 1–16. [4] K. Saha, S.S. Agasti, C. Kim, X. Li, V.M. Rotello, Gold nanoparticles in chemical and biological sensing, Chem. Rev. 112 (2012) 2739–2779. [5] P.N. Dyachenko, S.V. Karpeev, E.V. Fesik, Y.V. Miklyaev, V.S. Pavelyev, G.D. Malchikov, The three-dimensional photonic crystals coated by gold nanoparticles, Opt. Commun. 284 (2011) 885–888. [6] S. Yan, S. Zhang, Y. Lin, G. Liu, Electrocatalytic performance of gold nanoparticles supported on activated carbon for methanol oxidation in alkaline solution, J. Phys. Chem. C 115 (2011) 6986–6993. [7] N.S. Abadeer, C.J. Murphy, Recent progress in cancer thermal therapy using gold nanoparticles, J. Phys. Chem. C 120 (2016) 4691–4716. [8] F. Tian, F. Bonnier, A. Casey, A.E. Shanahan, H.J. Byrne, Surface enhanced Raman scattering with gold nanoparticles: effect of particle shape, Anal. Methods 6 (2014) 9116–9123. [9] X. Ji, X. Song, J. Li, Y. Bai, W. Yang, X. Peng, Size control of gold nanocrystals in citrate reduction: the third role of citrate, J. Am. Chem. Soc. 129 (2007) 13939– 13948. [10] Y. Sun, B. Mayers, Y. Xia, Transformation of silver nanospheres into nanobelts and triangular nanoplates through a thermal process, Nano Lett. 3 (2003) 675– 679. [11] M. Luty-Blocho, K. Fitzner, V. Hessel, P. Lob, M. Maskos, D. Metzke, K. PacAawski, M. Wojnicki, Synthesis of gold nanoparticles in an interdigital micromixer using ascorbic acid and sodium borohydride as reducers, Chem. Eng. J. 171 (2011) 279–290. [12] D.-H. Chen, S.-H. Wu, Synthesis of nickel nanoparticles in water-in-oil microemulsions, Chem. Mater. 12 (2000) 1354–1360. [13] D.P. Stankus, S.E. Lohse, J.E. Hutchison, J.A. Nason, Interactions between natural organic matter and gold nanoparticles stabilized with different organic capping agents, Environ. Sci. Technol. 45 (2011) 3238–3244. [14] B.L.V. Prasad, S.I. Stoeva, C.M. Sorensen, K.J. Klabunde, Digestive-ripening agents for gold nanoparticles: alternatives to thiols, Chem. Mater. 15 (2003) 935–942. [15] J. Piella, N.G. Bastas, V. Puntes, Size-controlled synthesis of sub-10-nanometer citrate-stabilized gold nanoparticles and related optical properties, Chem. Mater. 28 (2016) 1066–1075. [16] B. Kim, S.L. Tripp, A. Wei, Self-organization of large gold nanoparticle arrays, J. Am. Chem. Soc. 123 (2001) 7955–7956. [17] L.O. Brown, J.E. Hutchison, Formation and electron diffraction studies of ordered 2-D and 3-D superlattices of amine-stabilized gold nanocrystals, J. Phys. Chem. B 105 (2001) 8911–8916. [18] H. Yao, O. Momozawa, T. Hamatani, K. Kimura, Stepwise size-selective extraction of carboxylate-modified gold nanoparticles from an aqueous suspension into toluene with tetraoctylammonium cations, Chem. Mater. 13 (2001) 4692–4697. [19] F.K. Canitez, M.S. Yavuz, R. Ozturk, One-pot synthesis of gold nanoparticles using tetradentate porphyrins, J. Nanoparticle Res. 13 (2011) 7219–7228. [20] S. Rucareanu, M. Maccarini, J.L. Shepherd, R.B. Lennox, Polymer-capped gold nanoparticles by ligand-exchange reactions, J. Mater. Chem. 18 (2008) 5830– 5834.
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Regular Article
Polyoxovanadate fabricated gold nanoparticles: Application in SERS Bharat Baruah ⇑, Toni-Ann Miller Department of Chemistry and Biochemistry, Kennesaw State University, Kennesaw, GA 30144-5591, United States
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 25 September 2016 Accepted 15 October 2016 Available online 17 October 2016 Keywords: Gold nanoparticles Polyoxometalate Surface modification SERS Analyte
a b s t r a c t This article reports a surface modification of gold nanoparticles with water soluble polyoxometalate, V10O6 28 (decavanadate, V10). Two sizes of citrate-capped gold nanoparticles AuNP-Citrate-S (11 nm) and AuNP-Citrate-L (46 nm) were modified with V10 in aqueous media to form AuNP-V10-S and AuNP-V10-L, respectively. Both AuNP-V10-S and AuNP-V10-L were found to be significantly better than their citrate counterparts in strengthening Raman vibrational signals of analyte molecule. All the nanoparticles were characterized by UV–visible and Fourier transform infrared (FTIR) spectroscopies, dynamic light scattering (DLS), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) line analysis. We demonstrate that AuNP-V10-L is excellent surface-enhanced Raman scattering (SERS) substrate for a Raman-active analyte molecule at nanomolar concentrations. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction Nanoparticles are considered intermediate between bulk material and individual atoms and known to exhibit unique sizedependent characteristics [1]. In the current decade, nanoparticles have attracted enormous scientific interest due to the striking physical and chemical properties which can be altered by tweaking their configuration, morphology, size and diverse synthetic proce⇑ Corresponding author. E-mail address: [email protected] (B. Baruah). http://dx.doi.org/10.1016/j.jcis.2016.10.036 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
dures [2]. In addition, notable attention has been dedicated to gold nanoparticles (AuNPs) ascribable to their extensive prospective application in biosensing [3], chemical sensing [4], photonics [5], electrocatalysis [6], cancer therapy [7] and surface-enhanced Raman scattering (SERS) detection [8]. Nanostructured gold is synthesized by using a variety of reducing agents, namely, sodium citrate [9], sodium borohydride [10], ascorbic acid [11], hydrazine [12], etc. In nanoparticles synthesis, utilization of capping agent or ligand is common and are predominantly used to avoid aggregation and ensure monodispersed nanoparticles [13]. Amongst metal nanoparticles gold nanoparticles are straightforward to synthesize
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and relatively stable [14]. However, achieving monodispersed gold nanoparticles in the absence of commonly used citrate [15] or alkylthios [16] is often challenging [14]. Despite the fact various capping agents namely, amines [17], carboxylates [18], porphyrins [19], polymers [20], dendrimers [21], protein [22], and DNA [23] are used as a capping agent to stabilize gold nanoparticles. Recently, polyoxometalate capped nanoparticles have been reported [24–28]. Polyoxometalates (POM) are well defined metal-oxygen cluster anions of early-transition metals, commonly having d0 or d1 electronic configurations (e.g., V(V), Nb(V), Mo(VI) and Mo(V), or W(VI)), bridged by oxygen atoms (formally O2, or occasionally HO, ions) [24,26]. Ernst et al. demonstrated chemisorption of Keggin-type phosphododecamolybdate (PMo12O3 40 , PMo12) on AuNPs surfaces by partial replacement of alkanothiolatemodified AuNPs [29]. First synthesizing AuNPs achieved the PMo12-AuNP in toluene and then performing ligand-place exchange and phase transfer to aqueous solutions [29]. These PMo12-AuNP subsequently immobilized on glassy carbon and further fabricated with a polymer (PANI) [29]. The fabricated glassy carbon electrode containing POM, conducting polymer and AuNPs used for various electrocatalytic applications [29]. Zoladek and co-workers [28] developed and described a unique chemical method of fabrication of phosphomolybdate (PMo12O3 40 , PMo12) stabilized AuNPs. The uniform AuNP with a monolayer of phosphomolybdate used as support for immobilized electrocatalytic platinum particles. The composite performed efficient electrooxidation of ethanol [28]. Cheng et al. [24] demonstrated the formation of PMo12-capped AuNPs by the chemical synthesis. They have further shown that PMo12-capped AuNPs can be immobilized on ITO electrode with the help of PVP. This monolayer and multilayer PMo12-capped AuNPs modified ITO electrodes exhibited an excellent catalytic reduction of iodate [24]. A series of recent work by Wang and co-workers [25–27,30] exhibited in-situ formation of POM capped AuNPs revealing influence of metal cation size on the stability of POM monolayer on gold nanoparticles [25,27]. Furthermore, lability and reversible binding of the POM ligand on gold nanoparticle surface would facilitate undeviating access of the substrate molecules to the gold atoms on the AuNPs surface and thereby ensure optimum catalytic activity [30]. In the present work, we demonstrate two-step synthesis of AuNPs stabilized by a polyoxometalate anion called decavanadate, V10O6 28 (V10) (Fig. 1) which is stable in aqueous solution from pH 3 to 6 [31]. We first synthesized citrate-capped AuNPs, AuNP-Citrate [32,33] and then modified to V10-capped AuNPs, AuNP-V10 by ligand displacement method [34]. Two different sizes of AuNP-Citrate are prepared and further altered to AuNP-V10. We
characterized these nanoparticles by UV–visible and Fourier Transformed Infrared (FTIR) spectroscopies and dynamic light scattering (DLS), transmission electron microscopy (TEM), and energy dispersive (EDX) line analysis techniques. The SERS activity of these nanoparticles assessed with Raman spectroscopy. 2. Materials and methods 2.1. Materials NaAuCl42H2O (99%) and crystal violet (CV) (99%) were purchased from Sigma-Aldrich; trisodium citrate dihydrate (98%), ammonium metavanadate (98%), and hydroquinone (98%) were obtained from Fisher Scientific. All chemicals and solvents were used without further purification. Glassware were rinsed with aqua regia (3:1 v/v HCl (37%)/HNO3 (65%) solutions) and then rinsed thoroughly with DI H2O before use. Caution: aqua regia solutions are dangerous and highly corrosive. Aqua regia should be used with extreme care. Fresh aqua regia solutions should not be stored in closed containers. The DI water in all experiments was Milli-Q water (18 MX cm, Millipore). 2.2. Synthesis of citrate-capped small gold nanoparticles (AuNPCitrate-S) Citrate-capped nanoparticles were synthesized following literature methods [32,33]. 300 lL of 1.0% NaAuCl42H2O was placed in an Erlenmeyer flask that contained 30 mL of DI water. The mixture was brought to boiling rapidly while stirring. When the mixture came to a boil, 900 lL 1.0% sodium citrate was added. A series of color change was observed from blue, dark purple to final wine red. Thus formed AuNP-Citrate-S was purified by centrifuging at 12,000 rpm for 20 min in 1.5 mL batches in 1.5 mL eppendorf tubes. The supernatant was discarded, and the sample was redispersed in 1.5 mL pure DI water. The centrifugation as above was repeated twice, and nanoparticles were stored for further application. 2.3. Preparation of V10 capped small gold nanoparticles (AuNP-V10-S) Small V10-capped nanoparticles were prepared by replacing the citrate form the surface of AuNP-Citrate-S synthesized above. AuNP-Citrate-S was centrifuged for 20 min at 12,000 rpm. The pH of the solution was adjusted to 4.8 by adding 10 mM hydrochloric acid. To the 10 mL of pH adjusted AuNP-Citrate-S solution, 500 lL
Fig. 1. The molecular structure of decavanadate ion, V10O6 28 .
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of 100 mM ammonium metavanadate solutions at pH 3.8 (10 mM V10) was added. The solution was centrifuged at 10,000 rpm for 15 min in 1.0 mL batches in 1.5 mL eppendorf tubes. After that the supernatant was discarded and the sample was redispersed in 1.0 mL pure DI water, and 50 lL of V10 was added to each 1.0 mL batch. The centrifugation as above was repeated twice and final AuNP-V10-S thus formed were stored for further application. 2.4. Synthesis of citrate-capped large gold nanoparticles (AuNPCitrate-L) In the synthesis of large citrate-capped AuNPs, 100 lL of 1% NaAuCl42H2O solution was added to 7420 lL of DI water, in a 20 mL vial. To the 20 mL vial, 378 lL of the above prepared AuNP-Citrate-S solution was added as nanoparticle seed. The mixture was then stirred rapidly at room temperature for five minutes. Then, 22 lL of a 1% sodium citrate solution, immediately followed by 100 lL of 30 mM hydroquinone solution was added. The vial was allowed to stir for 10 min while reduction of gold salt took place to form an extra layer gold on the seed nanoparticles rendering large AuNP-Citrate-L [33]. Thus formed AuNP-Citrate-L was purified by centrifuging at 6000 rpm for 20 min in 1.0 mL batches in 1.5 mL eppendorf tubes. The supernatant was discarded, and the sample was redispersed in 1.0 mL pure DI water. The centrifugation as above was repeated twice, and nanoparticles were stored for further application. 2.5. Preparation of V10 capped large gold nanoparticles (AuNP-V10-L) Large V10-capped nanoparticles were prepared by following the same synthetic procedure as stated above for AuNP-V10-S. The only difference is, AuNP-Citrate-L was used as a precursor. The solution was centrifuged at 4000 rpm for 10 min in 1.0 mL batches in 1.5 mL eppendorf tubes after adding 100 lL of 100 mM ammonium metavanadate solutions at pH 3.8 (10 mM V10) was added to each 1.0 mL batch. After that, the supernatant was discarded, and the sample was redispersed in 1.0 mL pure DI water. The centrifugation as above was repeated twice and final AuNP-V10-L thus formed were stored for further application. 2.6. UV–visible and Fourier transform infrared spectroscopic measurement of nanoparticles The absorption spectrum was recorded using a Cary 4000 UV– visible spectrophotometer. FTIR measurement was performed using Perkin-Elmer FTIR Spectra 100 spectrometer fitted with diamond ATR. 2.7. Raman spectroscopic measurement The CV containing AuNP-Citrate-S, AuNP-Citrate-L, AuNP-V10S, and AuNP-V10-L colloids are well monodispersed in pure DI water. 2.5 lM and 50 lM solutions of CV in methanol are used as stock. Typical samples for SERS contained 1.0 mL of colloids dispersed in pure DI water and an aliquot of 4.0 or 20 lL of 2.5 lM CV (in MeOH) so that the final concentration of CV are 10 or 50 nM; an aliquot 2.0 or 5.0 or 10 lL of 50 lM CV (in MeOH) so that the final concentration of CV are 100 or 250 or 500 nM. Samples are allowed to stand for 1 h before Raman spectroscopic measurements are carried out using a DeltaNu Advantage 200A Raman spectrometer. This instrument is equipped with a HeNe laser set at 632.8 nm. The integration time for all measurements was 10 s. The Raman spectrum of 10 mM CV was acquired with an integration time of 1.0 s. However, the spectral resolution of 10 mM CV was weak as
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its Raman signal sits on top of a strong fluorescence background [35].
2.8. Electron microscopy and EDX analysis Transmission electron microscopy (TEM) morphologies were gauged with Hitachi H-7500 transmission electron microscope at an accelerating voltage of 75 kV, Hitachi H-7600 transmission electron microscope at an accelerating voltage of 120 kV and Hitachi H-9500 HR-TEM microscope with LaB6 source of resolution 0.1 nm at an accelerating voltage of 300 kV. Nanoparticle samples prepared in DI water and were centrifuged and redispersed in DI water. These solutions were filtered with 200 nm syringe filters before applying on grids for TEM measurements. Samples were prepared by spreading a 3.0 lL of the colloidal sample on an ultrathin 300 mesh Formvar/carbon-coated copper grid, dried in air. Energy dispersive X-ray (EDX) line analysis was performed using Scanning Transmission Electron Microscope (STEM) HD2000 with Field Emission source of resolution 0.24 nm at 200 kV.
2.9. Dynamic light scattering (DLS) measurements The mean hydrodynamic diameter of AuNP-Citrate-S, AuNPCitrate-L, AuNP-V10-S, and AuNP-V10-L colloids were determined using a commercial Zetasizer (Malvern Zetasizer Nano ZS, Malvern Instruments). Samples were loaded into disposable cells and measured the particle sizes twice and in triplicate. Zeta potential measured in a disposable cell with a dip probe.
3. Results and discussion It is evident from current research that polyoxometalate has been used by researchers to stabilize gold nanoparticles [24,26,28,29]. POM-AuNP prepared by chemical synthesis [24,28], ligand replacement [29], and in-situ synthesis [25,27,30]. We have modified two different sizes of AuNP-Citrate into the corresponding AuNP-V10 by applying simple ligand displacement method [29] in aqueous solutions. In a typical synthesis colloids of AuNPCitrate treated with an aqueous solution of V10 and further centrifuged to get rid of displaced citrate ions. This process repeated few times to ensure complete displacement of citrate by V10. Scheme 1 demonstrates the synthetic route described above. Scheme 1 also depicts the formation of AuNP-Citrate-L from AuNP-Citrate-S by seed growth method. Both AuNP-V10-L and AuNP-V01-S colloids are better SERS substrates compared to their citrate counterparts AuNP-Citrate-L and AuNP-Citrate-S, respectively.
3.1. Characterization of colloids by UV–visible spectroscopy Fig. 2A represents the UV–visible spectra of all the four different colloids. AuNP-Citrate-S and AuNP-Citrate-L show surface plasmon resonance bands at absorption maximum of 519 and 529 nm, respectively. The AuNP-Citrate-L synthesized from AuNP-CitrateS by seed growth method [33]. Apparently, the red shift of the UV–visible absorption maximum from AuNP-Citrate-S to AuNPCitrate-L support the size increase in nanoparticles. Similarly, the AuNP-V10-S and AuNP-V10-L show SPR bands at absorption maximum of 525 and 533 nm, respectively. Fig. 2A inset exhibits the digital photographs of all four colloid samples.
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Scheme 1. Syntheses of AuNP-Citrate-S, AuNP-Citrate-L, AuNP-V10-S, and AuNP-V10-L colloids.
Fig. 2. (A) UV–visible spectra and (B) dynamic light scattering size distribution of nanoparticle samples: (a) AuNP-citrate-S (black line), (b) AuNP-V10-S (red line), (c) AuNPcitrate-L (blue line), and (d) AuNP-V10-L (pink line). In the inset, we show digital photographs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. Size and zeta potential of nanoparticles with dynamic light scattering Fig. 2B shows particle size distribution (PSD) of AuNP-Citrate-S, AuNP-V10-S, AuNP-Citrate-L, and AuNP-V10-L measured using dynamic light scattering (DLS). AuNP-citrate-S displays a mean hydrodynamic diameter of 11.0 nm (average PDI = 0.220). We prepare AuNP-V10-S colloids by displacement of citrate in AuNPCitrate-S with V10 solution. AuNP-V10-S remain monodispersed with a mean hydrodynamic diameter of 14.0 nm (average PDI = 0.325) (Fig. 2B). We synthesize AuNP-Citrate-L from AuNPCitrate-S by seed growth method [33] and then modify further to AuNP-V10-L following the same method described above. Both AuNP-Citrate-L and AuNP-V10-L persist monodispersed in water with hydrodynamic diameter of 46.0 nm (average PDI = 0.125) and 51.0 nm (average PDI = 0.320), respectively. The sizes of these nanoparticles are also measured from the TEM images using imageJ software, and the measurements are comparable to those obtained from DLS measurements. The size distribution histograms are shown in Supplementary material (Fig. S1). As listed in Table 1 the electrophoretic mobility measurements for the above four colloid samples was done by zeta potential values obtained using commercial Zetasizer. The stability of the dispersed nanoparticles depends on their surface charge and the pH [36]. We measured zeta potential of all four colloid samples at pH 4.8 to verify their stability. Zeta potentials for AuNP-Citrate-S, AuNP-V10-S, AuNP-Citrate-L, and AuNP-V10-L colloids are 32.0 ± 2.0, 36.0 ± 2.0, 33.0 ± 2.0, and 35.0 ± 2.0, respectively.
Table 1 Mean hydrodynamic diameter and zeta potential of colloidal solutions. Colloidal samples
Size (d, nm)
Zeta potential (mV)
AuNP-Citrate-S AuNP-V10-S AuNP-Citrate-L AuNP-V10-L
11.0 ± 1.0 14.0 ± 1.0 46.0 ± 1.0 51.0 ± 1.0
32.0 ± 2.0 36.0 ± 2.0 33.0 ± 2.0 35.0 ± 2.0
The negative zeta potentials indicate negatively charged surface ligand in these colloid samples. 3.3. TEM images and EDX analysis of nanoparticles The morphologies of the nanoparticles were imaged using TEM. The TEM images of nanoparticles AuNP-Citrate-S, AuNP-V10-S, AuNP-Citrate-L, and AuNP-V10-L presented in Fig. 3A–D, respectively. The particle size distributions obtained (Fig. S1) from these images are comparable to those calculated by DLS in solution. The elemental composition of all four nanoparticles determined using EDX measurement with a Hitachi HD-2000 STEM at an operating voltage of 200 kV. In the EDX spectra of all four samples, the signature signal for gold appears at 2.15 keV (Fig. S2) [37]. For sample AuNP-V10-S (Fig. S2B) and AuNP-V10-L (Fig. S2D), one additional peak appears near 5.0 keV indicating the presence of V atom. This signal corresponds to previously reported EDX value for V atom [38]. Additional background signals near 0.13 and 8 keV are due to C and Cu present in Formvar/carbon and coated copper grid, respectively.
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Fig. 3. TEM images of (A) AuNP-Citrate-S (scale bar 100 nm), (B) AuNP-V10-S (scale bar 50 nm), (C) AuNP-Citrate-L (scale bar 100 nm), and (D) AuNP-V10-L (scale bar 50 nm) samples. Images were taken with a Hitachi H-7500 transmission electron microscope at an accelerating voltage of 75 kV (image A), Hitachi H-7600 transmission electron microscope at an accelerating voltage of 120 kV (image C) and Hitachi H-9500 HR-TEM Microscope at an accelerating voltage of 300 kV (images B and D). Aqueous sample stretch out on an ultrathin 300 mesh Formvar/carbon film on a copper grid and dried in air.
3.4. Characterization of colloids by FTIR spectroscopy We present the FTIR spectra in Fig. 4. Spectrum (a) depicts stretching frequencies for sodium citrate [39] at 3449/3244 cm1,
2966/2925 cm1, 1581 cm1, 1387 cm1, 1080/949 cm1 and 752 cm1 are attributed to free/H-bonded OAH, CAH stretch, C@O stretch, OAH in-plan bending, CAO stretch, and out-of-plan OAH bending. Spectrum (b) shows the stretching frequencies for V10 [40] at 3162/3033 cm1, 1405 cm1, 956 cm1, 831 cm1, and 744 cm1 attributed to OAH stretching, OAH bending, V@O stretch, asymmetric stretching of bridging VAOAV, and symmetric stretching of bridging VAOAV. As our nanoparticle samples and V10 stock solutions are acidic pHs (pH 4–5), we expect to have 3 H2V10O4 28 and H3V10O28 species in AuNP-V10-S and AuNP-V10-L [31]. For the nanoparticles, the samples were first dried on the ATR slot and then FTIR data were collected. For AuNP-Citrate-L and AuNP-Citrate-S we observe 2922/2857 cm1, 1563 cm1, and 1082 cm1 attributed to the CAH stretch, C@O stretch, and CAO stretch, respectively. The presence of these signals indicate that citrate is on the AuNPs surface. On the other hand for AuNP-V10L and AuNP-V10-S samples we observe 3190/3061 cm1, 1424 cm1, 962 cm1, 842 cm1, and 739 cm1 attributed to the OAH stretch, OAH bending, V@O stretch, asymmetric VAOAV stretching, and symmetric VAOAV stretching, respectively. The presence of characteristic V@O and VAOAV vibrational signals in both AuNP-V10-L and AuNP-V10-S colloids indicate that the nanoparticles surface has capped with V10. 3.5. AuNP-citrate and AuNP-V10 as SERS substrates
Fig. 4. FTIR spectra of (a) solid trisodium citrate (black line), (b) V10O6 28 (V10) (red line), (c) AuNP-Citrate-L (blue line), (d) AuNP-V10-L (orange line), (e) AuNP-CitrateS (cyan line), and (f) AuNP-V10-S (pink line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Dispersed gold nanoparticles can carry [41] the surface plasmon resonance band even in the presence of a diverse organic analyte molecule, and hence these are satisfactory SERS substrates. Very recently, Lai et al. fabricated [42] mesostructured polyoxometalate(POM)-silica surfactant (PSS) template containing 3D densely spaced AuNPs of 2 nm [42]. This new composite material display notable SERS enhancement with adsorbed 4-mercaptobenzoic acid due to the presence of 3D nanometer-spaced AuNPs [42]. Another
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study reported the size- and shape-controlled synthesis of complex silver dendrites based on galvanic displacement reaction assisted by the presence of a POM [43]. The POM both served as reductant and stabilizer and these POM based silver dendrite exhibited strong SERS signal for Rhodamine B [43]. The AuNP-Citrate-L, AuNP-Citrate-S, AuNP-V10-L, and AuNP-V10-S nanoparticles, synthesized in the current work were assessed for SERS using crystal violet (CV), a Raman-active analyte. We observed a dramatic difference in SERS activity among the four colloids mentioned above. The SERS activity of the above four colloids is examined using CV as the Raman-active analyte. Fig. 5A shows SERS spectra AuNP-Citrate-L samples in the presence of CV by varying the concentration from 0.0 nM to 500 nM using a laser excitation wavelength of 633 nm. The SERS signal of CV is noticeable only in the sample of AuNP-Citrate-L with 500 nM CV (Fig. 5A (f), pink line). All of these samples were prepared by adding calculated amount of CV stock solutions in MeOH to AuNP-Citrate-L. After that, the AuNP-Citrate-L samples are incubated for 1 h before SERS measurements. The SERS signals of CV are allocated [44] as strong ring CAC stretching at 1621 cm1, N-phenyl stretching at 1383 cm1, ring CAH bending at 1173 cm1, medium (CCcenterC) at 916 cm1, and medium signal at 803 cm1. The SERS signals are slightly shifted from the Raman vibrational modes at 1619, 1378, 1169, 913, and 796 cm1 reported in the literature [44,45]. Fig. 5B shows the corresponding of surface plasmon resonance bands of all of the above colloids of AuNP-Citrate-L in presence and absence of CV. Sample (f) (the pink line) has an extra peak at 725 nm attributed to aggregation of AuNP-Citrate-L in the presence of 500 nM CV. Fig. 5B inset depicts digital photographs of all these samples with and without CV. Fig. 6A shows the SERS signal of CV with AuNP-V10-L in the presence of CV by varying the concentration from 0.0 nM to 500 nM. The SERS signal of CV is noticeable for the sample of AuNP-V10-L with 50 nM to 500 nM CV (Fig. 6A (c)–(f)). All of these samples were prepared by adding calculated amount of CV stock solutions in MeOH to AuNP-V10-L. After addition of CV aliquot, the AuNP-V10-L samples are incubated for 1 h before SERS measurements. The SERS signals of CV are allocated [44] as strong ring CAC stretching at 1620 cm1, N-phenyl stretching at 1374 cm1, ring CAH bending at 1174 cm1, medium (CCcenterC) at 917 cm1, and medium signal at 805 cm1. The SERS signals are slightly shifted from the Raman vibrations at 1619, 1378, 1169, 913, and 796 cm1 reported in the literature [44,45]. Fig. 6B represents the plot of SERS intensity versus concentration of CV for the most intense Raman active signals namely
1174, 1374, and 1620 cm1. As the concentration increases the SERS intensity increases. Raman mode at 1620 cm1 and 1374 cm1 have equal intensities and the mode at 1174 cm1 has the lowest intensity for 500 nM CV. For the other samples with 50 nM CV to 250 nM CV, all three have comparable intensities. Fig. 6C shows surface plasmon resonance bands of all of the above colloids of AuNP-V10-L in presence and absence of CV. There is no sign of nanoparticle aggregation even in the presence of 500 nM CV. Fig. 6C inset shows digital photographs of all these samples with and without CV. Fig. 7A shows the SERS signal for AuNP-V10-S and AuNPCitrate-S in the presence of CV at 500 nM. It is clear that in the presence of AuNP-V10-S we observe the typical SERS modes of CV at 1180, 1377, and 1620 cm1. However, in the presence of AuNP-Citrate-S, we do not observe any of those signals. Both samples were prepared as explained above. In Fig. 7B we present the UV–visible spectra of AuNP-Citrate-S and AuNP-V10-S. To evaluate SERS performance, we determine the analytical enhancement factor (AEF) [46]. AEF of a nanoparticles sample is determined from the ratio of SERS intensity (ISERS) of a vibrational mode in question of the selected analyte and the corresponding Raman intensity (IRS) under equivalent experimental settings (e.g. laser power, integration time, sample preparation, etc.) using the equation [46]:
AEF ¼
ISERS C RS IRS C SERS
ð1Þ
CRS and CSERS are the concentrations of the analyte in the Raman and SERS experiments, respectively. For spectrum (f) of Fig. 5A, CSERS = 5.0 10–7 M, and ISERS (1621 cm–1) = 5618. The corresponding CRS = 0.01 M and IRS (1621 cm–1) = 1489. The AEF was calculated to be 7.6 104. For spectrum (f) of Fig. 6A, CSERS = 5.0 10–7 M, and ISERS (1620 cm1) = 19,204 and the AEF was calculated to be 2.6 105. This value is close to the highest AEF documented in the literature for metal-based nanoparticles [46]. Similarly, for spectrum (b) of Fig. 7A, CSERS = 5.0 10–7 M, and ISERS (1620 cm–1) = 2212 and the AEF was calculated to be 3.0 104. In Table 2 we summarize the AEF for all the samples for the most intense Raman shift near 1174, 1377, and 1620 cm1. All the nanoparticle samples have a negative charge at the surface, and the analyte molecule CV has a positive change. Electrostatically, they would attract each other. As indicated in Table 2 for AuNP-V10-L colloids, the Raman mode at 1620 cm1 has the highest AEF of 2.6 105 and Raman mode at 1374 cm1 has minimum AEF of 1.4 105. AuNP-Citrate-L samples follow the same trend.
Fig. 5. (A) SERS spectra of AuNP-Citrate-L in presence of (a) 0.0 nM CV (black line), (b) 10 nM CV (red line), (c) 50 nM CV (blue line), (d) 100 nM CV (orange line), (e) 250 nM CV (cyan line), and (f) 500 nM CV (pink line); (B) UV–visible spectra are shown for all the above AuNP-Citrate-L samples. Inset shows the corresponding digital photographs of these colloids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. (A) SERS spectra of AuNP-V10-L in presence of (a) 0.0 nM CV (black line), (b) 10 nM CV (red line), (c) 50 nM CV (blue line), (d) 100 nM CV (orange line), (e) 250 nM CV (cyan line), and (f) 500 nM CV (pink line); (B) The corresponding plots of Raman intensity versus concentrations are for the most intense Raman active modes (1620, 1374 and 1174 cm1) of CV; (C) UV–visible spectra are shown for all the above AuNP-V10-L samples. Inset shows the corresponding digital photographs of these colloids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. (A) SERS spectra of (a) AuNP-Citrate-S in the presence of 500 nM CV (black line) and (b) AuNP-V10-S in the presence of 500 nM CV (red line); (B) UV–visible spectra are shown for both colloids solutions. Corresponding digital photographs are in the inset. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 2 List of Raman shift, SERS intensity, Raman dye concentration and analytical enhancement factor (AEF) of CV in the presence of AuNP-Citrate-L, AuNP-V10-S, and AuNP-V10-L. Sample
Raman shift (cm1)
SERS intensity
CV (nM)
AEF
AuNP-Citrate-L AuNP-V10-S AuNP-V10-L AuNP-Citrate-L AuNP-V10-S AuNP-V10-L AuNP-Citrate-L AuNP-V10-S AuNP-V10-L
1173 1180 1174 1382 1377 1374 1621 1620 1620
6720 4051 17,894 6439 4191 20,076 5618 2212 19,204
500 500 500 500 500 500 500 500 500
6.4 104 5.4 104 1.6 105 5.2 104 5.6 104 1.4 105 7.6 104 3.0 104 2.6 105
The colloidal systems AuNP-V10-L has enhancement factor in order of 105 and AuNP-Citrate-L has enhancement factors in order of 104. Even AuNP-V10-S has enhancement factor in order of 104. 4. Conclusion In conclusion, we report the fabrication of AuNP-V10-L and AuNP-V10-S by simple ligand displacement reaction of AuNPCitrate-L and AuNP-Citrate-S, respectively. These colloids are highly stable in aqueous solution and can be stable in the presence of analyte molecules. AuNP-V10-L is 5 nm larger, and AuNP-V10S is 3 nm greater than their corresponding precursors. Red-shift in UV–visible spectra, dynamic light scattering size distribution
and TEM images support this size growth. Red shift in SPR band and FTIR data support surface modification of AuNP by V10. EDX line analysis (Supporting information) reinforce the presence of V-atom in AuNP-V10-S and AuNP-V10-L colloids. In this work we have also shown that AuNP-Citrate-L and AuNP-V10-L nanoparticles augment the Raman signal of adsorbed analyte molecule. The analytical enhancement factor of SERS for AuNP-Citrate-L and AuNP-V10-L are in order of 104 and 105, respectively. AuNPCitrate-L yield SERS signal at 500 nM analyte concentration, whereas AuNP-V10-L enables SERS signal of the analyte at as low as 50 nM. On the other hand, AuNP-V10-S enable SERS signal at 500 nM analyte concentration, but AuNP-Citrate-S do not yield any signal at that concentration. Further research is in progress.
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