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2018
Polyethylenimine-assisted
seed-mediated synthesis of gold
nanoparticles for surface-enhanced Raman scattering studies
Philip, Anish
Elsevier BV
Tieteelliset aikakauslehtiartikkelit
© Elsevier B.V.
CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/
http://dx.doi.org/10.1016/j.apsusc.2018.03.042
https://erepo.uef.fi/handle/123456789/6612
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Full Length Article
Polyethylenimine-assisted seed-mediated synthesis of gold nanoparticles for surface-enhanced Raman scattering studies
Anish Philip, Bright Ankudze, Tuula T. Pakkanen
PII: S0169-4332(18)30702-5
DOI: https://doi.org/10.1016/j.apsusc.2018.03.042
Reference: APSUSC 38790
To appear in: Applied Surface Science Received Date: 27 September 2017 Revised Date: 18 January 2018 Accepted Date: 5 March 2018
Please cite this article as: A. Philip, B. Ankudze, T.T. Pakkanen, Polyethylenimine-assisted seed-mediated synthesis of gold nanoparticles for surface-enhanced Raman scattering studies, Applied Surface Science (2018), doi: https://
doi.org/10.1016/j.apsusc.2018.03.042
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Polyethylenimine-assisted seed-mediated synthesis of gold nanoparticles for surface- enhanced Raman scattering studies
Anish Philipa, Bright Ankudzea, Tuula T. Pakkanena ,*
a Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101, Joensuu, Finland.
* Corresponding Author: tuula.pakkanen@uef.fi, tel. +358504354379.
Abstract
Large-sized gold nanoparticles (AuNPs) were synthesized with a new polyethylenimineassisted seedmediated method for surface-enhanced Raman scattering (SERS) studies. The size and polydispersity of gold nanoparticles are controlled in the growth step with the amounts of polyethylenimine (PEI) and seeds. Influence of three silicon oxide supports having different surface morphologies, namely halloysite (Hal) nanotubes, glass plates and inverse opal films of SiO2, on the performance of gold nanoparticles in Raman scattering of a 4-aminothiophenol (4-ATP) analyte was investigated. Electrostatic interaction between positively charged polyethylenimine-capped AuNPs and negatively charged surfaces of silicon oxide supports was utilized in fabrication of the SERS substrates using deposition and infiltration methods. The Au-photonic crystal of the three SERS substrate groups is the most active one as it showed the highest analytical enhancement factor (AEF) and the lowest detection limit of 1x10-8 M for 4-ATP. Coupling of the optical properties of photonic crystals with the plasmonic properties of AuNPs provided Au-photonic crystals with the high SERS activity. The AuNPs clusters formed both in the photonic crystal and on the glass plate are capable of forming more hot spots as compared to sparsely distributed AuNPs on Hal nanotubes and thereby increasing the SERS enhancement.
Keywords: Gold nanoparticles, polyethylenimine, seed-mediated synthesis, SERS, halloysite nanotubes, photonic crystals
1. Introduction
Gold nanoparticles (AuNPs) have drawn attention of researchers worldwide because of their wide applicability [1–6]. There are several factors such as the size, shape, nature of stabilizing agent and external charges determining the applications of AuNPs [2,3,7]. While small nanoparticles with large number of active sites are essential for catalysis, bigger nanoparticles with rough edges are more useful in surface-enhanced Raman scattering (SERS) applications [7–11]. Enhancement of Raman signals by gold nanoparticle substrates and the ability of AuNPs to form hot spots are presently an active field of research [12–14]. The SERS method has the advantage to increase the intensity of weak Raman signals by a factor of 105 – 1014, that enables detection of analytes in trace amounts and even a single molecule detection [15].
The Raman signal enhancement of an analyte molecule adsorbed on a nanostructured substrate surface is possible via two different mechanisms, electromagnetic and chemical mechanisms [12,15,16]. According to the electromagnetic mechanism, the signal enhancement occurs via interaction of the excited localized surface plasmon resonance of a metallic nanoparticle with an analyte molecule close to the nanoparticle surface [15,16]. The chemical mechanism operates through the charge transfer between the analyte molecule and the metallic nanoparticle [15,16].
SERS has found wide applications in the selective and sensitive detection of trace amounts of target molecules in food science [17,18], narcotics [6,19], health science [3,18,20], biology [21,22] and environmental pollutants [23,24].
Both chemical and physical methods have been adopted for the fabrication of SERS substrates [6,12,13,25–27]. There are several types of materials available to support metallic nanoparticles in the SERS substrates such as glass plates, nanofibers, photonic crystals and halloysite nanotubes.
Substrates based on photonic crystals (inverse opal films) are of interest as coupling of photonic band gap properties with a localized surface plasmon resonance (SPR) of metallic nanoparticles can
provide highly sensitive SERS substrates [12,28–30]. The extra benefit of using photonic crystals with metallic nanoparticles is, that cavities of the inverse opal structure can assist formation of metal nanoparticle clusters and thereby creation of more hot spots [30].
Naturally available halloysite (Hal) nanotubes can also be used as supporting materials for SERS substrates [13]. Hal nanotubes (Al2Si2O5(OH)4. nH2O) are two-layered (1:1) aluminosilicates, which are chemically similar to a kaolinite mineral with interlayer water molecules [13,31]. Hal nanotubes are generally 0.2 to 2 μm in length with an outer diameter of 40-70 nm [31]. Hal nanotubes have aluminol groups (Al-OH) and silanol groups (Si-OH) on their internal and external surfaces, respectively [13,31]. Properties such as a high surface area and the different charges on the external and internal surfaces make halloysites an unique nano-structured support for several applications [13,31].
Nowadays, the seed-mediated synthesis is considered to be the best approach for preparation of AuNPs with an uniform size and shape [32–34]. With this method, it is possible to produce highly mono-dispersed gold nanoparticles having a wide range of sizes [11,32]. Most of these synthesis procedures developed for seed-mediated synthesis of AuNPs are citrate based methods [10,32,33,35,36]. Since the charge of gold nanoparticles is an important factor in electrostatic interaction based adsorption, the seed-mediated synthesis of AuNPs using a suitable stabilizing agent is essential.
In our previous studies [7,37], we have demonstrated that polyethylenimine-capped (PEI-capped) gold nanoparticles can be easily used to decorate various silicon oxide support materials by using electrostatic interaction. In this new study, SERS properties of PEI-capped gold nanoparticles are studied using three different silicon oxide supports, such as halloysite nanotubes, a silicon dioxide photonic crystal film (inverse opal) and glass plate. Large-sized AuNPs required for the SERS
application are obtained via a novel PEIassisted seed-mediated growth method. Influence of the polyethylenimine polymer on the size control of the AuNPs in a seed-mediated synthesis is demonstrated. PEI can stabilize gold nanoparticles by capping them via its amino groups and being a cationic polyelectrolyte PEI can furnish AuNPs with positive charges. The large-sized AuNPs are obtained in the growth step of the seed-mediated synthesis procedure by using PEI and PEI-capped gold seeds in the presence of ascorbic acid reducing agent (Scheme 1A). In fabrication of SERS substrates, AuNPs are bound on the silicon oxide materials utilizing electrostatic interaction and capillary forces in the deposition (nanotubes and glass plate) and infiltration (inverse opal films) of the nanoparticles (Scheme 1B). The SERS performance of the prepared Au-Hal nanotubes and Au- photonic crystal substrates is studied in the low concentration level detection of 4-aminothiophenol (4-ATP).
Scheme 1. Schematic presentation of the synthesis of large-sized AuNPs via a seed-mediated growth method (1A) and fabrication of SERS substrates using different silicon oxide materials (1B).
2. Experimental
2.1. Materials
HAuCl4·3H2O (Alfa Aesar, ≥ 99.99%), sodium borohydride (NaBH4, Sigma Aldrich, ≥ 99%), ascorbic acid (Sigma Aldrich, ≥ 99%) and branched polyethylenimine (PEI, Sigma Aldrich, Mw = 25,000) were used for the synthesis of gold nanoparticles. Tetraethyl orthosilicate (TEOS, Sigma Aldrich, 98%), hydrochloric acid (HCl, VWR Chemicals, 37%), ethanol (Altia, 99.5%) were used for the preparation of silica precursor. 4-Aminothiophenol (4-ATP, Aldrich, ≥97%) and methanol (VWR Chemicals) were used for preparation of a SERS analyte sample. Halloysite nanoclay (Al2Si2O5(OH)4 · 2H2O, Sigma Aldrich) were used without further purifications. Ammonium persulfate (APS, Sigma Aldrich, ≥ 98%), sodium dodecyl sulfate (SDS, Sigma Aldrich, ≥ 98.5%) and styrene (Sigma Aldrich, ≥ 99%) were used for polystyrene spheres synthesis. Polystyrene spheres (PS spheres, 347±4 nm) prepared following a literature procedure[37] were used as a template in preparation of an inverse opal structure of SiO2 (See Procedure S1 in Supporting Information). Glass plates (VWR) were used as a support for fabrication of photonic crystals, Au- glass and Au-Hal substrates. Glass plates were treated with piranha solution for 30 minutes before using them in the fabrication of photonic crystals. After the piranha treatment, the glass plates were washed with a large amount of water and finally with ethanol. They were then air-dried before the use. Deionized water was used in all experiments.
2.2. Seed-mediated synthesis of gold nanoparticles (AuNPs) 2.2.1. Synthesis of AuNPs seeds
A solution of 0.5 mM HAuCl4·3H2O in deionized water (15 mL) was stirred in a 25 mL round- bottomed flask under ice cold conditions. To this gold salt solution, 10 µL of 0.6 mM of PEI was added under stirring. The reaction mixture was mixed for 5 minutes and then 600 µL of 10 mM of
ice cold solution of NaBH4 was added. The resulting solution was stirred for another 15 minutes prior to use.
2.2.2. Seeded growth of AuNPs
In a general procedure, 60 mL of 0.5 mM HAuCl4·3H2O in deionized water was stirred in a 100 mL erlenmeyer flask. To this stirring solution different volumes of seed solution (given in Table 1) was added. After stirring for 2 minutes, different volumes of 0.6 mM of PEI was added (Table 1) to the reaction mixture. After stirring the reaction mixture for another 2 minutes, 3 mL of 10 mM of ascorbic acid was added. The resulting growth solution was stirred for another 30 minutes before depositing nanoparticles on Hal nanotubes.
Table 1. Volumes of PEI and seed solutions used in the growth step of the seed-mediated synthesis of different sized AuNPs.
Sample Volume of 0.6mM PEI (µL) Volume of seed (µL) AuNP 25-100
AuNP 50-100 AuNP 75-100 AuNP 50-200 AuNP 75-200 AuNP 75-400 AuNP 150-400 AuNP 500-1000
25 50 75 50 75 75 150 500
100 100 100 200 200 400 400 1000
2.3. Preparation of Au-Hal nanotube SERS substrates
The Hal nanotubes with different sized AuNPs were prepared by following a deposition method described in our earlier work [7]. In a general procedure, 50 mg of Hal nanotubes was added to 60 mL of AuNPs solution and stirred for 15 minutes. The mixture was then centrifuged at a speed of 5300 rpm for 5 minutes. Au-Hal nanotubes were separated by decanting the clear supernatant from the mixture. The precipitate thus obtained was washed with water and centrifuged. The washing- decantation procedure was repeated three times to purify the Au-Hal nanotubes. The Au-Hal nanotube substrates were dried at 105°C for 10 hours.
2.4. Fabrication of an Au-SiO2 photonic crystal and Au-glass plate SERS substrates 2.4.1. Preparation of a photonic crystal of SiO2
A photonic crystal was fabricated using a previously reported procedure [37]. In a general procedure, a silica precursor solution comprising of TEOS, ethanol and HCl in a ratio of 1:1.5:1 was stirred for one hour before use. A 52 μL volume of as prepared silica precursor was added to 150 μL of PS spheres dispersed in 20 mL of water. This mixture was then stirred for 10 minutes before immersing a piranha-treated glass plate vertically in it. The plate was then oven dried at 62°C for 32 hours. An opal film of SiO2 and PS were formed as the solvent evaporated. The SiO2
photonic crystals were obtained by calcining these opal films at 450°C for 4 hours.
2.4.2. Infiltration of AuNPs in SiO2 photonic crystal and deposition of AuNPs on a glass plate An infiltration procedure was adopted for depositing AuNPs into the voids of photonic crystal. For infiltration procedure, the growth solution of AuNPs were centrifuged for 10 minutes at 5300 rpm.
Isolated AuNPs were redispersed in 60 ml ethanol solution. The plate having a SiO2 photonic crystal film was placed vertically in 20 ml of the AuNPs solution (AuNP 75-100) in the fume hood.
After the infiltration, the Au-SiO2 photonic crystal substrate was washed with ethanol and dried under air. The Au-glass plate substrate was prepared in a similar procedure by immersing a piranha- treated glass plate in 20 ml of the redispersed AuNPs solution (AuNP 75-100), followed by washing with ethanol and air drying.
2.5. Characterization methods
The morphology of AuNPs, Au-Hal nanotubes, photonic crystals, Au-photonic crystals, and Au- glass plates were analyzed using a Hitachi S-4800 FE-SEM scanning electron microscope. For the SEM measurement the sample was placed on a carbon tape and analyzed at a voltage of 10 kV and a current of 15 mA. An energy dispersive X-ray spectroscopy (EDS, Thermo Electron Corporation) using the Noran System six (NSS) was used for the elemental identification of samples. The
scanning transmission electron microscopy (STEM) was used for the size distribution measurement of AuNPs at a voltage of 30 kV and a current of 25 mA. For the STEM analysis, gold nanoparticles were deposited from a water solution. In a general procedure, a small amount of AuNPs sample was mixed with water and a small drop of it was placed on a copper grid coated with Lacey carbon films. After drying at ambient temperature for 5h, the copper grids were subjected to analysis.
A Perkin Elmer Lambda 900 UV/Vis/NIR spectrometer was used to measure the absorbance of gold nanoparticles in aqueous solutions. A Perkin Elmer Lambda 900 UV/Vis/NIR spectrometer fitted with a 150 mm integrating sphere was used to measure the reflectance spectra of photonic crystals and Au-photonic crystals.
A Renishaw® inVia Raman microscope with wireTM 3.4 software was employed for measuring the SERS spectra of 4-aminothiophenol. For the SERS measurement, an excitation laser of 785 nm with a laser power of about 3 mW and an integration time of 10 s was used. The Au-Hal nanotubes (1 mg) was sonicated with 20 μL of a 4-ATP solution in a micro-centrifuge tube for 3 minutes and then incubated for 60 minutes. After incubation, the analyte solution containing Au-Hal nanotubes was transferred on a microscopic slide and the slide was dried at ambient temperature prior to the analysis. The Au-SiO2 photonic crystal and Au-glass plate substrates were incubated in a methanol solution of 4-ATP for 60 minutes in a refrigerator to avoid solvent evaporation and subsequently dried in air before the analysis.
2.6. Prediction of stop band position
Prediction of stop band or the Bragg’s diffraction wavelength ( was done using the combined Bragg’s and Snell’s law [37–39] according to equations 1 and 2
(1)
where is the interplanar spacing between the hkl planes and it is given by
(2)
where, m is the order of Bragg’s diffraction (normally its value is 1), D is the pore spacing, navg is the average refractive index of the photonic crystals and θ is the angle of incidence. The spherical pores of SiO2 photonic crystals are assumed to be arranged in a face-centered-cubic (FCC) structure with (111) as the main plane [37]. The volume fraction of air and SiO2 in the photonic crystal structure is taken as 74% and 26%, respectively [37–39]. The refractive indices for air and silica used in the calculation are 1.000 and 1.455 respectively [38,39]. The θ angle is taken as 8° as the sample was tilted 8° from the surface normal during the reflectance measurements.
2.7. Calculation of analytical enhancement factor (AEF) The AEF was calculated using the following equation [40]
(3)
where ISERS is the intensity of 4-ATP measured using a SERS substrate, IRaman is the intensity of 4- ATP recorded on the substrate in the absence of AuNPS. CSERS and CRaman are concentrations of 4- ATP responsible for ISERS and IRaman, respectively. PSERS and PRaman are laser powers used for detection of ISERS and IRaman, respectively.
3. Results and discussion
3.1. PEI-assisted seed-mediated synthesis of gold nanoparticles
Formation of gold nanoparticle seeds was indicated by physical changes in the gold salt solution upon addition of both polyethylenimine (PEI) and the reducing agent, NaBH4. Addition of PEI into the gold salt solution resulted in a color change from pale yellow to a more intense yellow color, which can be due to the interaction of AuCl4 ¯
anion with cationic branched PEI [41]. The role of PEI as a reducing and stabilizing agent in AuNPs synthesis is well known [42,43]. The absence of a surface plasmon resonance (SPR) in the UV-Vis spectrum of the gold salt solution containing PEI indicates that AuNPs are not formed by mere addition of PEI. Reduction of gold salt by PEI is completed only under heating conditions or on long time standing at room temperature [43]. The
AuNPs produced in the presence of mere PEI have been found to be polydispersed as compared to monodispersed nanoparticles produced using both PEI and NaBH4 [43]. In our case, NaBH4 reduced the gold salt, which was indicated by a color change from yellow to wine-red. Seeds produced by this method showed SPR at a wavelength of 518 nm and had a size distribution of 6±2 nm (Fig. S1 in Supporting Information).
In the growth step, volumes of PEI and seed solutions used had a direct influence on the size of AuNPs (Table 2). When the added volumes of PEI and seeds in the growth solution were low, a red shift of the surface plasmon resonance along with peak broadening was observed (Fig. 1). The red shift in the SPR wavelength and the broad peak shape are signs of an increase in the particle size and in the polydispersity of the nanoparticles obtained [10,43]. Ascorbic acid used in the growth step acts as a mild reducing agent, while gold seeds act as the nucleation centers. When the number of seeds is high, there will be more nucleation centers for the growth and thereby decrease in the size of resulting AuNPs [11].
By varying the amounts of PEI and seed solutions in the growth step, AuNPs obtained had the size ranging from 37±3 nm to 108±20 nm (Table 2). The size-dispersity of AuNPs decreased when higher amounts of seeds and PEI were used. Both seeds and PEI have an important role in controlling the size of AuNPs produced in the growth step. PEI helps in controlling the size of AuNPs probably by covering facets of growing nanoparticles and thus hindering further growth and enabling formation of AuNPs with a uniform spherical shape. Irregularities in the particle shapes were more prominent, when the amounts of PEI and seeds were low in the growth step. AuNPs produced with the lowest amount of PEI (AuNP 25-100) are showing rough edges as compared to the smooth surface of AuNPs produced with the higher volumes of PEI (Fig. 2).
Fig. 1. UV-Vis spectra of AuNPs solutions with maximum and minimum concentrations of PEI and seeds.
Table 2. The sizes and surface plasmon resonance (SPR) wavelengths of the synthesized AuNPs.
Sample Particle size (nm) LSPR wavelength (nm) AuNP 25-100
AuNP 50-100 AuNP 75-100 AuNP 50-200 AuNP 75-200 AuNP 75-400 AuNP 150-400 AuNP 500-1000
108±20 90±21 78±14 67±14 62±13 55±13 45±06 37±03
581 569 554 548 542 540 530 526
For particles without a spherical shape, the long axis has been used for the size calculation.
Fig. 2. STEM images of AuNPs produced in the growth step. (A) AuNP 25-100, (B) AuNP 50-100, (C) AuNP 75-100, (D) AuNP 50-200, (E) AuNP 75-200, (F) AuNP 75-400, (G) AuNP 150-400 and (H) AuNP 500-1000. Each scale bar is 150 nm.
3.2. Au-Hal nanotube SERS substrates
In fabrication of Au-Hal nanotube SERS substrates, an electrostatic interaction based adsorption was utilized in deposition of PEI capped AuNPs on the external surface of Hal nanotubes [7].
According to the SEM image (Fig. 3), AuNPs were mainly singly distributed on the surface of Hal nanotubes. The EDS spectrum of Au-Hal nanotubes also confirmed adsorption of AuNPs by showing signals for gold (Fig. 3). The size of AuNPs on Au-Hal nanotubes was ranging from 35±05 to 124±34 nm (see Table 3 and Fig. S2 in Supporting Information). The AuNPs obtained with smaller amounts of seeds and PEI showed a clear increase in the size distribution upon deposition on Hal nanotubes due to a drying treatment at 105 oC. The narrow size distribution of AuNPs observed in the Au-Hal nanotubes, having AuNPs prepared with higher amounts of PEI and seeds, can be attributed to the protective stabilization of AuNPs by the PEI capping agents against aggregation.
Fig. 3. Analytical details of Au-Hal nanotubes. (A) SEM image of Au-Hal 150-400 and (B) EDS spectrum of the same sample. The scale bar is 600 nm.
3.2.1. SERS performance of the Au-Hal nanotube substrates
The Au-Hal nanotube substrates were examined in the SERS detection of 4-aminothiophenol (4-
A B
ATP). A comparison between the SERS spectra of 4-ATP measured on an Au-Hal nanotube substrate (75-100) and the SERS spectrum of the mere substrate (Fig. 4A) shows that the most of Raman signals of 4-ATP are not overlapping with those of PEI present in the Au-Hal nanotube substrate. In Fig. 4, the vibrational bands at 1078 cm-1 and 1581 cm-1 are assigned to the C-S and C- C stretching of 4-ATP, respectively, while the band at 1179 cm-1 is assigned to the C-H bending of 4-ATP [12,44]. These a1-type vibrational modes, which are enhanced through the electromagnetic mechanism, are characteristic of 4-ATP [44]. The SERS spectrum of the mere Au-Hal substrate shows, that PEI does not give any vibrational bands at 1078 cm-1 and therefore use of PEI as a stabilizing agent in gold nanoparticles does not disturb the 4-ATP detection and analysis. Only the vibrational band of PEI at 1574 cm-1, due to amine bending, is interfering with the C-C stretching of 4-ATP at 1581 cm-1, but the intensity of the PEI band at a laser power used is considerably low.
Fig. 4. SERS spectra of 4-ATP using the Au-Hal nanotube 75-100 substrate. (A) Comparison of
SERS spectra of 4-ATP at different concentrations with that of an analyte free substrate. (B)
Twenty-five randomly measured SERS spectra of 10-4 M 4-ATP.
In SERS, the electromagnetic field enhancement produced by plasmonic metal nanoparticles is highly dependent on the number of electrons excited, and hence dependent on the size and size distribution of metal nanoparticles [45]. According to the earlier studies, the optimum size of metallic nanoparticles for a high SERS performance is 30-100 nm [45]. In order to understand influence of the AuNP size on the SERS activity of Au-Hal nanotubes, all the Au-Hal nanotube samples were examined in SERS analysis of 4-ATP (Fig. S3). The SERS results showed that the size of AuNPs has an effect on the SERS performance of Au-Hal nanotubes (Table 3). The analytical enhancement factors (AEFs) of different Au-Hal nanotubes show that AuNPs produced with moderate volumes of PEI and seeds in the growth stage are more suitable for SERS applications as compared to those synthesized with lower or higher amounts of PEI and seeds.
Those gold nanoparticles with the low PEI and seed amounts used in the growth step (Au-Hal 25- 100 and Au-Hal 50-100) showed a tendency to aggregate on Hal nanotubes, but their SERS activity remained low. Although their localized surface plasmon resonances of the AuNPs (Table 2) are closer to the excitation wavelength of the laser used (785 nm), the large radiation damping effect of bigger-sized AuNPs may result in decreased enhancement factors [45,46].
Table 3. Comparison of the AEF and RSD results of Au-Hal nanotubes with different sized AuNPs.
a Ananalytical enhancement factor (AEF) of the most prominent peak (peak at 1078 cm-1) using equation 3. b A relative standard deviation (RSD) is determined using the signal at 1078 cm-1 in the 25 randomly measured spectra.
All the SERS substrates showed an enhancement factor in the order of 105,but the Au-Hal nanotube substrate obtained with 75 µL of PEI and 100 µL of seeds had the highest AEF value (Table 3).
Comparison of 25 randomly measured SERS spectra of 4-ATP (Fig. 4B) on the Au-Hal 75-100 substrate and calculation of a relative standard deviation (RSD) of 10% for the signal at 1078 cm-1 indicate that the SERS measurements using the Au-Hal nanotube substrates are reproducible and consistent. The Au-Hal nanotubes having AuNPs prepared with the moderate quantities of PEI and seeds showed the best SERS performances, which can be attributed to their optimal gold particle size and plasmon resonance frequency [45,46].
The SERS analysis of 4-ATP using the Au-Hal nanotubes showed that these types of substrates are Sample Size of AuNPs on
substrate (nm)
AEF (x105)a RSD (%)b
Au-Hal 25-100 Au-Hal 50-100 Au-Hal 75-100 Au-Hal 50-200 Au-Hal 75-200 Au-Hal 75-400 Au-Hal 150-400 Au-Hal 500-1000
124±34 98±24 83±26 68±31 59±14 54±17 49±06 35±05
2 2 6 5 4 3 3 2
18 14 10 14 7 13 11 13
capable of detecting the analyte in concentrations as low as 1x10-6 M (Fig. S3). These results are comparable with the earlier observations made with an Au-Hal nanotube SERS substrate containing gold nanoparticles in size of 20 – 40 nm and having a LSPR at 537 nm [13]. Zhu et al. [13] were able to detect the rhodamine R6G analyte down to a concentration of 1x10-6 M, but only a few of the R6G analyte peaks were detected at this low concentration [13]. Using the present SERS substrate all of the characteristic peaks of 4-ATP were observed even at a concentration of 1x10-6 M. The reason for the better performance of the current Au-Hal SERS substrates can be the larger- sized AuNPs used.
3.3. Au-SiO2 photonic crystal SERS substrate
A SEM analysis of the photonic crystal of SiO2 showed a well-ordered pore structure with the size of the cavities in the range of 305±5 nm (Fig. 5). A single-cycle infiltration of gold nanoparticles (AuNP 75-100 solution) into cavities of the inverse opal structure caused organizing of nanoparticles in clusters [37] with a nanoscale separation between the particles. The presence of AuNPs in photonic crystals was verified with an EDS measurement (Fig. S4). Evaporation occurring at the solvent/air/substrate interface, along with the capillary force, results in assembly of AuNPs into clusters inside the cavities of the photonic crystal [37,47,48]. Around 2-10 gold nanoparticles were found in each clusters. Though some cavities had a single nanoparticle or no particles, their number was notably less compared to those having clusters.
Fig. 5. SEM images of photonic crystals of SiO2 before (A) and after (B) infiltration with AuNPs (Insert shows a magnified image of a single cavity of the Au-photonic crystal). The scale bar is 500 nm.
The position of stop band of the photonic crystal remained the same (Fig. 6) even after the infiltration of gold nanoparticles. The position of stop band or the Bragg’s diffraction wavelength predicted using Eqs. (1) and (2) was 553±7 nm, but the experimental wavelength observed was 482 nm. The deviation may be due to imperfections in the crystal lattice. The reflectance spectrum of the Au-photonic crystal sample (Fig. 6) showed a depression around 644 nm corresponding to the plasmon resonance of AuNPs. The LSPR signal had red-shifted from the value of 554 nm, observed for colloidal AuNPs. The size of individual AuNPs in the photonic crystal had increased slightly from 78 14 nm to 80±19 nm, but the size increment does not explain the large shift in LSPR. The red shift can be most likely attributed to clustering of AuNPs in the photonic crystal [12,16]. The LSPR of the Au-SiO2 photonic crystal is close to the excitation wavelength of the laser used, that would be expected to influence the SERS performance of this substrate.
Fig. 6. UV-Vis reflectance spectra of a photonic crystal of SiO2 before (dotted lines) and after (solid 0
10 20 30 40 50
300 400 500 600 700 800
Relectance (%)
Wavelength (nm)
Photonic crystal Au- Photonic crystal
LSPR
line) infiltration with AuNPs.
3.3.1. SERS performance of the Au-SiO2 photonic crystal substrate
The SERS analysis of 4-ATP using an Au-photonic crystal substrate (Fig. 7) gave a better result compared to those of Au-Hal nanotubes. With the Au-photonic crystal substrate it was possible to detect 4-ATP at a concentration as low as 10-8 M. The good SERS result can be attributed to the combination of the photonic properties of both the porous inverse opal support and the gold nanoparticle clusters. The repeated light scattering from the photonic crystal and the hot spots initiated by gold nanoparticles in the clusters will produce the high SERS activity of the Au- photonic crystal substrate [12,16]. Because of the high number of AuNP clusters in cavities, the amount of analyte trapped at the junctions of AuNPs can also be high, that in turn can increase the Raman signal intensity of analyte molecules.
The effectiveness of the Au-photonic crystal substrate was assessed with an AEF value of 2 x107. Similar enhancement factors and slightly lower detection limits were obtained with our earlier Au- photonic crystal SERS substrate containing a different capping agent, octadecylamine [12], using similar incubation times for 4-ATP. The low RSD value (12 %) of 25 random measurements (Fig.
S5) obtained for the 4-ATP signal at 1078 cm-1 indicates the high-level homogeneity of the present Au-photonic crystal substrate in the SERS enhancement.
Fig. 7. Comparison of SERS spectra of 4-ATP at different concentrations with that of an analyte free substrate using an Au-photonic crystal substrate.
3.4. SERS performance of the Au-glass plate substrate
The third SERS substrate (Au-glass plate substrate) was prepared by depositing gold nanoparticles from the AuNP 75-100 solution on a glass plate mostly as a monolayer of two-dimensional nanoparticle clusters (Fig. 8A). The conventional silanization procedure for anchoring AuNPs on glass plate [49,50] was replaced with an electrostatic interaction and solvent evaporation induced deposition of AuNPs at the solvent-air boundary of the support. The SEM images showed that AuNPs are closely arranged on the glass plate with nanoscale separations. Since the nanoparticles were stabilized with PEI, no aggregation was observed. The periodic arrangement of AuNPs with nano-sized gaps made possible a high electromagnetic field enhancement and increased the SERS intensity of the analyte molecules [16,46]. The SERS analysis of 4-ATP using the Au-glass plate substrate enabled the detection of analyte molecules down to a concentration of 10-7 M (Fig. 8B).
The effectiveness of the Au-glass plate substrate was evidenced with an AEF value of 9 x106.
Fig. 8. SEM image of Au-glass plate SERS substrate (A). Comparison of SERS spectra of 4-ATP at different concentrations with that of an analyte free substrate using an Au-glass plate SERS substrate (B).
In conclusion, the Au-SiO2 photonic crystal substrate involving synergetic effects of the photonic crystal and AuNP clusters was the best SERS performer among the three SERS substrates (Fig. 9).
In the case of the Au-photonic crystals and Au-glass plate substrates, nanoscale separations (1-15 nm) observed between the gold nanoparticles in the clusters are evidence of SERS hotspots, where the electromagnetic field is greatly enhanced, thus promoting SERS performance of this type of substrates [16].
Fig. 9. SERS analysis of 1x10-6 M of 4-ATP using the three different SERS substrates.
4. Conclusions
In this work, the assistance of cationic polyethylenimine (PEI) in the seed-mediated synthesis of gold nanoparticle was studied for the first time. With this synthesis method it is possible to produce AuNPs with a size range between 37±03 nm and 108±20 nm by varying the amounts of PEI and seeds in the growth step. Three types of SERS substrates, where synthesized AuNPs were supported on different silicon oxide support materials such as, photonic crystal of SiO2 , glass plate or halloysite nanotubes, were studied in the SERS analysis of 4-aminothiophenol (4-ATP).
Influence of photonic properties of the three types of substrates on the SERS enhancement is clearly shown in the AEF values and detection limits. The clustering of AuNPs along with the optical properties of photonic crystals provided the Au-photonic crystal substrate with a high AEF of 2x107 and a very low detection limit of 1x10-8 M of 4-ATP. The ability of Au-glass plate and Au-Hal nanotubes as SERS substrates was demonstrated with a detection limit of 1x10-7 M and 1x10-6 M of 4-ATP respectively. Our studies showed that vibrational bands of PEI do not interfere with the main Raman signals of the 4-ATP molecule and therefore use of PEI as a capping agent for AuNPs is appropriate. A minor disadvantage of the highly sensitive Au-photonic crystal substrates is the multi-step fabrication procedure. Naturally available halloysite nanotubes however offer a straightforward route to moderately sensitive, robust SERS substrates, since AuNPs can be directly deposited on nanotubes using PEI as a coupling agent.
Acknowledgment
Scholarships to two of the authors (A.P. and B.A.) from University of Eastern Finland is gratefully acknowledged.
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Graphical abstract
Highlights
Polyethylenimine-assisted seed-mediated synthesis of gold nanoparticles presented.
Size and polydispersity of AuNPs controlled by amounts of PEI and seeds in growth.
Three different silicon oxide materials used to support AuNPs in SERS substrates.
Electrostatic interaction utilized in binding gold nanoparticles to the supports.
AuNP clusters in photonic crystal produced the highest SERS activity.
