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Langmuir 2007, 23, 9836-9843
Synthesis of Positively Charged Silver Nanoparticles via Photoreduction of AgNO3 in Branched Polyethyleneimine/HEPES Solutions Siliu Tan,† Melek Erol,‡ Athula Attygalle,‡ Henry Du,*,† and Svetlana Sukhishvili*,‡ Department of Chemical, Biomedical, and Materials Engineering and Department of Chemistry and Chemical Biology, SteVens Institute of Technology, Hoboken, Castle Point on Hudson, New Jersey 07030 ReceiVed April 27, 2007. In Final Form: June 15, 2007 Branched polyethyleneimine (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were used collaboratively to reduce silver nitrate under UV irradiation for the synthesis of positively charged silver nanoparticles. The effects of molar ratio of the ingredients and the molecular weight of BPEI on the particle size and distribution were investigated. The mechanism for the reduction of Ag+ ions in the BPEI/HEPES mixtures entails oxidative cleavage of BPEI chains that results in the formation of positively charged BPEI fragments enriched with amide groups as well as in the production of formaldehyde, which serves as a reducing agent for Ag+ ions. The resultant silver nanoparticles are positively charged due to protonation of surface amino groups. Importantly, these positively charged Ag nanoparticles demonstrate superior SERS activity over negatively charged citrate reduced Ag nanoparticles for the detection of thiocyanate and perchlorate ions; therefore, they are promising candidates for sensing and detection of a variety of negatively charged analytes in aqueous solutions using surface-enhanced Raman spectroscopy (SERS).
1. Introduction Silver and gold colloidal nanoparticles have been broadly exploited for ultra-trace chemical and biological sensing and detection by means of surface-enhanced Raman spectroscopy (SERS). The most common approach for the preparation of SERSactive silver and gold nanoparticles is the chemical reduction of the constituent salts. Typical chemical reducing agents include sodium citrate,1 sodium borohydride,2 hydrazine,3 and hydroxylamine hydrochloride.4 Generally, the resultant nanoparticles are negatively charged due to the adsorption of negatively charged ions on their surfaces. However, such nanoparticles are not well suited for SERS-based detection of anions due to the unfavorable condition for anion adsorption on the particle surface stemming from electrostatic repulsion. One approach to render such particles useful for the detection of anions is the neutralization of the anion charge by mixing them with positively charged molecules prior to applying them to silver or gold nanoparticle surface. The decrease in electrostatic repulsion between the anions and the negatively charged nanoparticles results in increased anion adsorption and SERS sensitivity, as demonstrated by Yea et al. by mixing cyanide anions with polyamine spermine tetrachloride molecules for SERS detection using citrate-reduced silver nanoparticles.5 Another approach to concentrate anions in the proximity of the nanoparticle surface is to modify the surface of the underlying substrate with -N+(CH3)3 and -NH3+/-NH2 groups.6 Still one more widely explored strategy to make negatively charged nanoparticles suitable for detection of * Corresponding authors. E-mail:
[email protected]. Tel: 1-201216-5544; E-mail:
[email protected]. Tel: 1-201-216-5262. † Department of Chemical, Biomedical, and Materials Engineering. ‡ Department of Chemistry and Chemical Biology. (1) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391-3395. (2) Boesch, S. E.; York, S. S.; Frech, R.; Wheeler, R. A. Phys. Chem. Comm. 2001, 4, 1-10. (3) Nickel, U.; Mansyreff, K.; Schneider, S. J. Raman Spectrosc. 2004, 35, 101-110. (4) Leopold, N.; Lendl, B. J. Phys. Chem. B 2003, 107, 5723-5727. (5) Yea, K. H.; Lee, S.; Kyong, J. B.; Choo, J.; Lee, E. K.; Joo, S. W.; Lee, S. Analyst 2005, 130, 1009-1011. (6) Wang, W.; Ruan, C.; Gu, B. Anal. Chim. Acta 2006, 567, 121-126.
negatively charged analytes is to modify their surface with cationic functional groups. Examples of such surface modification techniques include the attachment of amino or quaternary ammonium groups on silver or gold surfaces via the use of aliphatic7-9 or aromatic cationic thiols.8,9 The use of polyamines to prepare silver and gold nanoparticles was also employed by others earlier.10-16 For example, gold nanoparticles were prepared using alkylamine molecules,12 primary amine containing hyperbranched molecules,10 linear polyethylenimine (LPEI),16,17 poly(allylamine hydrochloride),16 N-[3-(trimethoxysilyl)propyl]polyethylenimine16 or linear alkylated PEI14 at elevated temperatures, and branched polyethylenimine (BPEI) at room temperature14 or at elevated temperature.16 In these experiments, polyamines played the role of both reducing and stabilizing agents. Preparation of silver nanoparticles using LPEI17 or via the reaction of an insoluble silver myristate and a tertiary alkylamine solvent12 were also reported. However, the reduction process employed for those studies required either heating or a long reaction time. 4-(2-Hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer has also been used as a reducing agent for gold18,19 and silver20,21 nanoparticles. (7) Ruan, C.; Wang, W.; Gu, B. Anal. Chim. Acta 2006, 567, 114-120. (8) Mosier-Boss, P. A.; Lieberman, S. H. Appl. Spectrosc. 2003, 57, 11291137. (9) Mosier-Boss, P. A.; Lieberman, S. H. Appl. Spectrosc. 2000, 54, 11261135. (10) Rybak, B. M.; Ornatska, M.; Bergman, K. N.; Genson, K. L.; Tsukruk, V. V. Langmuir 2006, 22, 1027-1037. (11) Yamamoto, M.; Nakamoto, M. J. Mater. Chem. 2003, 13, 2064-2065. (12) Aslam, M.; Fu, L.; Su, M.; Vijayamohanan, K.; Dravid, V. P. J. Mater. Chem. 2004, 14, 1795-1797. (13) Kuo, P. L.; Chen, C. C.; Jao, M. W. J. Phys. Chem. B 2005, 109, 94459450. (14) Sun, X.; Dong, S.; Wang, E. J. Colloid Interface Sci. 2005, 288, 301303. (15) Sun, X.; Luo, Y. Mater. Lett. 2005, 59, 3847-3850. (16) Sun, X.; Dong, S.; Wang, E. Mater. Chem. Phys. 2006, 96, 29-33. (17) Sun, X.; Dong, S.; Wang, E. Polymer 2004, 45, 2181-2184. (18) Habib, A.; Tabata, M.; Wu, Y. G. Bull. Chem. Soc. Jpn. 2005, 78, 262269. (19) Habib, A.; Tabata, M. J. Inorg. Biochem. 2004, 98, 1696-1702. (20) Sun, R. W. Y.; Chen, R.; Chung, N. P. Y.; Ho, C. M.; Lin, C. L. S.; Che, C. M. Chem. Commun. 2005, 5059-5061.
10.1021/la701236v CCC: $37.00 © 2007 American Chemical Society Published on Web 08/17/2007
Synthesis of PositiVely Charged Ag Nanoparticles
We report, for the first time, UV-assisted reduction of Ag+ to Ag0 in polyamine solutions at low concentration of HEPES that yields positively charged Ag nanoparticles (denoted as Ag [+]). Besides studying the effects of molar ratio of ingredients and the molecular weight of BPEI on the particle size and distribution by means of UV-visible absorption spectroscopy and scanning electron microscopy (SEM), we explored the chemical transformations in BPEI:AgNO3:HEPES aqueous solutions during UV-induced formation of silver nanoparticles in the presence of oxygen using Fourier transform infrared spectroscopy (FTIR), mass spectrometry, X-ray photoelectron spectroscopy (XPS), and ζ-potential measurements. In contrast to previous studies which suggested a redox reaction between amino groups of the polyamine and Ag+ or AuCl4- ions,10,13,14,16 we showed that severe chain cleavage of branched polyethyleneimine as well as formation of amide groups on the BPEI fragments occurred during the silver nanoparticle synthesis. These results were substantiated with mass spectrometry and FTIR results, respectively. We suggest a mechanism which involves reduction of AgNO3 with formaldehyde, a side product of the cleavage reaction.22 We also presented the evidence that the cleavage reaction and the rate of formation of silver nanoparticles are coupled and were significantly enhanced in the presence of oxygen. The BPEI fragments containing both primary amino and amide groups provide nanoparticles with positive charge when bound at the surface of silver nanoparticles. Very significantly, Ag [+] nanoparticles demonstrated superior sensitivity in SERS detection of perchlorate and thiocyanate over commonly used negatively charged citrate-reduced nanoparticles. 2. Experimental Section 2.1. Materials. The following reagents were purchased from the indicated suppliers and used without further purification: N-(2hydroxyethyl)piperazine-N′-2-ethanesulfonic acid (HEPES, reagent grade, Fisher Scientific), silver nitrate (ultrapure grade, Acros), branched polyethyleneimine (BPEI) with the molecular weights of 1200, 10 000, and 70 000 g‚mol-1 (Polysciences, Inc.), sodium citrate (enzyme grade, Fisher Scientific), polystyrene sulfonate sodium salt (PSS) with the molecular weight of 500 000 g‚mol-1 (Scientific Polymer Products, Inc), sodium thiosulfate (99.99+%, Aldrich), sodium sulfide (Aldrich), sodium perchlorate (99+%, Acros), and sodium thiocyanate (99.99+%, Sigma-Aldrich). Potassium bromide (KBr) powder of spectroscopic grade was obtained from International Crystal Labs. Water used for the experiments was filtered with Barnstead ion-exchange columns and then further purified by passage through Milli-Q (Millipore) deionizing and filtration columns. N-type (100) silicon wafers with a thickness of 650 µm and resistivity of 7500-9500 Ω cm were purchased with the item number of B694 from El-Cat Inc., NJ. All glassware and silicon wafers were cleaned in Nochromix (Godax Laboratories, Inc., MD) solution in concentrated sulfuric acid overnight, followed by thorough rinsing with Milli-Q water. 2.2. Synthesis of Ag [+] Nanoparticles. BPEI and AgNO3 were separately dissolved in 0.1 mM HEPES to give the desired concentrations. The two solutions were then mixed (50:50 by volume) to give the final BPEI:AgNO3:HEPES molar ratio ranging from 1:1:0.1 to 0.5:1:0.1. Chemical structures of BPEI and HEPES are given in Figure 1. Note that the molar concentration of BPEI repeating units was used throughout this paper. The glass vials used during the experiments were precoated with BPEI to form a positively charged surface in order to suppress the adsorption of BPEI in the reaction mixture to the negatively charged walls of the glass vials. The formation of silver nanoparticles was initiated by the exposure (21) Tan, S.; Pristinski, D.; Sukhishvili, S.; Du, H. Proc. SPIE 2005, 6008, art. no. 600808. (22) Idris, S. A.; Mkhatresh, O. A.; Heatley, F. Polym. Int. 2006, 55, 10401048.
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Figure 1. Chemical structures of (a) N-(2-hydroxyethyl) piperazineN′-2-ethanesulfonic acid (HEPES) and (b) branched polyethyleneimine (BPEI). Scheme 1. Preparation of Type I Substrates Using BPEI/ HEPES Reduced Ag [+] Nanoparticles
of the final mixture to a UV lamp (BHK Mercury Grid Lamps), which is a standard low-pressure mercury arc lamp providing uniform, high intensity, short wave UV irradiation at all low-pressure mercury lines including the ozone producing 185 nm line, for 120 min. A total of 10 mL of BPEI/AgNO3/HEPES mixture in a glass vial of a diameter of 2 cm and a height of 5 cm was placed under UV lamp at a distance of ∼6 cm. In the first 5 min of irradiation, the initially colorless solution started to develop a faint yellow color, indicating the formation of silver nanoparticles. As irradiation continued, more nanoparticles were formed and the solution continued to darken in color. The completion of the reduction of Ag+ to Ag0 was tested using a procedure described by Aymonier et al.23 Specifically, Ag [+] nanoparticles were filtered out, and aqueous solutions of Na2S2O3 and Na2S were subsequently added to the supernatant. When residual Ag+ ions were present in the supernatant solution, a black precipitate was observed. The presence of excess BPEI was tested by the reaction of the supernatant solution with a strong polyanion, polystyrene sulfonate (PSS). If BPEI was present in solution, turbidity was observed. These control experiments proved the complete consumption of Ag+ ions during the synthesis. To determine the concentration of the Ag nanoparticles in solution, the average particle size was first evaluated from the SEM micrographs using image analysis software (Digimizer by MedCalc Software, Belgium) as described in section 2.4.1e. Given complete consumption of Ag+ ions upon reduction of Ag+ to Ag0, and from the known density of silver (10.5 g/cm3) and initial concentration of Ag+ ions, the number of Ag nanoparticles per mL Ag colloid solution can be calculated as CAg+/[F(4/3)π r3], where CAg+ is the concentration of silver ions in g/mL, F is the density of silver in g/cm3, and r is the average radius of Ag nanoparticles in cm. The particle density was determined to be 2 × 1010 particles/mL for Ag [+] colloidal solution with BPEI:AgNO3:HEPES molar ratio of 1:1: 0.1. 2.3. Nanoparticle Immobilization on Oxidized Silicon Substrates. Two types of substrates were prepared. Type I substrates were prepared by the adsorption of BPEI/HEPES reduced nanoparticles (Ag [+]) on an oxidized silicon substrate by immersing it in Ag [+] colloidal solution with a particle density of 2 × 1010 particles/mL at pH 5 for 4 h. Ag [+] nanoparticles used for the preparation of type I substrates were synthesized at BPEI:AgNO3: HEPES molar ratio of 1:1:0.1, and had an average particle size of 33 ( 8 nm. As shown in Scheme 1, silver nanoparticles were attached to the oxidized silicon substrates by electrostatic attractions. After nanoparticle immobilization, the substrates were thoroughly rinsed (23) Aymonier, C.; Schlotterbeck, U.; Antonietti, L.; Zacharias, P.; Thomann, R.; Tiller, J. C.; Mecking, S. Chem. Commun. 2002, 8, 3018-3019.
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Scheme 2. Preparation of Type II Substrates Using Citrate Reduced Ag [-] Nanoparticles and BPEI
with Milli-Q water without dislodging the nanoparticles according to SEM. The coverage density of nanoparticles on the oxidized silicon substrate is approximately 20 particles/µm2. It can be controlled using silver colloids of different particle concentrations or via changing the adsorption time. Scheme 2 shows the preparation of type II substrates. First, negatively charged silver nanoparticles, Ag [-], with average particle size of 56 ( 15 nm were prepared using Lee and Meisel method.1 Briefly, 90 mg of AgNO3 was dissolved in 500 mL of H2O and brought to a boil. A solution of 1% sodium citrate (10 mL) was added, and the solution was kept boiling for 1 h. The resultant Ag [-] nanoparticles were attached on the BPEI-modified oxidized silicon substrate from a colloid solution with a 10-fold dilution in 10 mM HEPES buffer at pH 7 for 4 h. Intermediate BPEI (MW of 1200) layer was allowed to adsorb on the oxidized silicon substrate from a polymer solution of 0.2 mg/mL in 0.1 mM HEPES at pH 8.3 for 20 min. The excess polymer was then removed from the surface by rinsing with Milli-Q water. The coverage of Ag [-] nanoparticles on type II substrate was comparable to the Ag [+] nanoparticle coverage on type I substrate. Finally, to mimic the Ag [+] nanoparticles, another BPEI layer was coated onto the surface attached Ag [-] nanoparticles from a polymer solution of 0.05 mg/mL and pH 8.3 (the same concentration and molecular weight as used in the reduction of Ag [+] nanoparticles using a molar ratio for BPEI: AgNO3:HEPES of 1:1:0.1) for 20 min, and again the excess polymer was removed from the surface by rinsing with Milli-Q water. All of the oxidized silicon substrates used for sample preparation for XPS investigation were first precleaned with soap, then soaked into Nochromix solution in concentrated sulfuric acid overnight, and finally thoroughly rinsed with Milli-Q water. 2.4. Analytical Measurements. 2.4.1. Characterization of SilVer Nanoparticles and Chemical Changes Occurring in the BPEI: AgNO3:HEPES Mixture during Reduction. Chemical changes occurring in the BPEI:AgNO3:HEPES mixture as a result of UV reduction were monitored using FTIR and mass spectrometry techniques. 2.4.1.a. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were recorded using a PARAGON 1000 PC FTIR spectrometer (Perkin-Elmer). For FTIR analysis, 20 mL of 1.0 mg/mL BPEI in water and 20 mL of 1.0 mg/mL BPEI in 2 mM HEPES (at the same molar ratio used in the silver reduction step) with and without 2 h of UV irradiation were freeze-dried. The freeze-dried samples were mixed with KBr and compressed into a pellet for further spectra collection. 2.4.1.b. Mass Spectrometry. Three sample solutions of BPEI were prepared for mass spectrometric analysis: (1) 0.1 mg/mL BPEI in water without any treatment, (2) 0.1 mg/mL BPEI in water exposed to UV for 120 min under argon gas, and (3) 0.1 mg/mL BPEI in water exposed to UV for 120 min while open to air. The effect of UV on BPEI solution was investigated by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Mass spectrometric analysis was performed on a MALDITofspec-2E instrument (Micromass, U.K.) equipped with a pulsed nitrogen laser emitting 337 nm radiation. Pressure in the ionization chamber was maintained between 1 × 10-7 and 4 × 10-7 Torr. The matrix solution contained 10 mg/mL R-cyano-4-hydroxycinnamic acid (alpha-matrix) dissolved in 50/50 acetonitrile/water with 0.1% TFA. A total of 10 µL of matrix solution was added to 10 µL of BPEI solutions under different treatments, spotted on the MALDI target plate, and evaporated to dryness. Negative-ion spectra were recorded under reflectron conditions. 2.4.1.c. ζ-Potential Measurements. The ζ-potential values of Ag [+] or Ag [-] nanoparticles were measured by laser Doppler electrophoresis using Zetasizer Nano-ZS equipment (Malvern
Instruments Ltd). The results were averaged over 30 runs. Each value was obtained by averaging measurements of three samples. For the ζ-potential measurements, samples were prepared by mixing different concentrations of BPEI solutions with or without UV irradiation with Ag [-] nanoparticles at pH 7.6. The final concentration of Ag [-] nanoparticles in the mixture was 5 × 1012 particles/mL. After allowing the polymer to adsorb on Ag [-] nanoparticles for ∼15 min, ζ-potential values were measured. The saturation concentrations for the adsorption of UV treated and untreated BPEI on Ag [-] nanoparticles were estimated to be 4 × 10-3 and 6 × 10-4 mg/mL, respectively. For measurements at various pH values, Ag [-] nanoparticles were mixed with UV-treated and untreated BPEI solutions at their saturation concentrations and their ζ-potential values were measured after adjusting the solution pH in the range of 2-11.5. 2.4.1.d. UV-Visible Spectroscopy. The plasmon peaks of the silver nanoparticles were measured using UV-visible spectrophotometer (Hitachi U3000). Prior to measurements, the Ag [+] nanoparticle colloidal solution was diluted 3-fold, and the citratereduced Ag colloidal solution was diluted 10-fold with Milli-Q water. 2.4.1.e. Scanning Electron Microscopy (SEM). The Ag [+] nanoparticle size and coverage were characterized using SEM (LEO 982) after Ag [+] nanoparticles were attached to silicon substrates as described above. The nanoparticle coverage and size distribution was determined using Digimizer image analysis software developed by MedCalc Software (Mariakerke, Belgium). To determine the average particle size, typically 150 nanoparticles were analyzed. The size ranges determined by image analyses of SEM micrographs are reported in section 3.1. TEM images of the Ag [+] nanoparticles were also taken (data not shown); although Ag [+] nanoparticles of ∼5 nm in diameter were occasionally observed in TEM micrographs, their occurrence was very sparse and inconsequential. 2.4.2. X-ray Photoelectron Spectroscopy (XPS). The chemical state of the elements on the surface of type I and type II substrates was analyzed by a multifunctional X-ray photoelectron spectroscope (PHI 5700 LSci). Preparation of type I and type II substrates was explained in section 2.3. The monochromated Al KR line was used as the excitation source. The binding energy at 284.8 eV for C 1s in hydrocarbon was used as the reference. 2.5. SERS Measurements. A home-built apparatus was used for the SERS measurements. Instrumental details are as follows. A 532 nm wavelength light beam from a Laserglow D1-532 laser was spatially filtered and expanded three times, band-pass filtered, reflected from a Chroma Q540LP dichroic mirror, and then illuminated the back aperture of an Olympus 40× objective, N.A. 0.85. The excitation light intensity in front of the objective was ∼10 mW. A sample cell, which was prepared according to one of the procedures described in section 2.3, filled with analyte solution as its bottom was situated on a Newport ULTRAlign 561D translation stage equipped with New Focus 8301 computer-controlled piezo actuators. SERS measurements were conducted using the same objective of ∼1.5 µm focusing diameter for excitation and spectral collection. The SERS signal from the objective passed through a dichroic mirror, was filtered by a Kaizer SuperNotch filter, and was focused by a collimator into a spectroscopic grade multimode fiber (Newport, 400 µm core). A fiber-coupled Acton SpectraPro 2300 spectrometer with Roper Scientific liquid nitrogen cooled CCD detector was used in spectrum acquisition. Data were processed using Origin 7 software. Glass tubes (Bellco Glass, 8 mm in outer diameter and 8 mm in height) were adhesively bonded at one end with glass slides (EMS, 22 × 22 × 0.15 mm) to form water-tight sample cells for immobilization of silver nanoparticles on the glass substrates (bottom of the cell) for their later use as SERS substrates. Prior to their use,
Synthesis of PositiVely Charged Ag Nanoparticles
Figure 2. UV-visible spectra of Ag [+] nanoparticles with BPEI: AgNO3:HEPES molar ratio of 0.5:1:0.1 after different durations of UV irradiation. Insets show SEM images of the Ag [+] nanoparticles on silicon substrates by BPEI/HEPES reduction after UV irradiation for (a) 30 min and (b) 90 min. The Ag [+] nanoparticles were allowed to adsorb on oxidized silicon substrates for 4 h at pH 5. the glass tubes and slides were cleaned by UV exposure for several hours, followed by soaking in Nochromix solution in concentrated sulfuric acid overnight, and finally by thorough rinsing with Milli-Q water. Two types of SERS substrates were prepared as described in section 2.3 except that type II substrates did not contain the BPEI coverage on top of the Ag [-] nanoparticles. For all SERS measurements, ∼0.2 mL of perchlorate (10-8-10-5 M or 1 ppb to1 ppm) or cyanide (3.8 × 10-8-3.8 × 10-5 M or 1 ppb to 10 ppm) aqueous solutions were added in the sample cells modified with either Ag [+] or Ag [-] nanoparticles. SERS spectra were collected with an exposure time of 100 s from two different positions for each substrate type investigated.
3. Results and Discussion 3.1. Synthesis of Ag [+] Nanoparticles. In this study, an efficient strategy to produce positively charged silver nanoparticles, Ag [+], is developed for the first time, to our knowledge, via synergistic use of BPEI and HEPES in the UV reduction of silver ions. We discuss here the parameters controlling the nanoparticle synthesis such as reaction time, molar ratio of the ingredients and molecular weight of BPEI. The time-dependent growth of Ag [+] nanoparticles with BPEI: AgNO3:HEPES molar ratio of 0.5:1:0.1 was characterized using UV-visible spectroscopy. Figure 2 shows an increase in the UV absorption peak intensity with UV irradiation time. Analysis of SEM images of Ag [+] nanoparticles adsorbed onto silicon wafers (insets in Figure 2) showed that there was no noticeable size change of Ag [+] nanoparticles with different irradiation time.
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The average particle size for Ag [+] particles produced with the molar ratio of 0.5:1:0.1 of BPEI:AgNO3:HEPES remained around 66 ( 11 nm. However, the intensity of the absorption peak of Ag [+] nanoparticles in solution and the number density of nanoparticles attached to a substrate experienced approximately a 10-fold increase when UV exposure time increased from 30 to 90 min. These results indicate that time of UV exposure affects the concentration of Ag [+] nanoparticles formed but have little effect on their size. In contrast, the BPEI:AgNO3:HEPES molar ratio had a dramatic effect on the nanoparticle size. Figure 3 shows the UV-visible spectra of Ag [+] nanoparticles formed from aqueous solutions at three different BPEI:AgNO3:HEPES molar ratios of 0.5:1:0.1, 0.75:1:0.1, and 1:1:0.1 as well as the corresponding SEM images of the nanoparticles immobilized on silicon substrates. Note that in these experiments, HEPES and AgNO3 concentrations were kept constant. SEM images showed that an increase in the BPEI to Ag ratio results in a decrease in the particle size. The average Ag [+] nanoparticle sizes were 66 ( 11, 46 ( 11 and 33 ( 8 nm for the BPEI:AgNO3:HEPES molar ratios of 0.5:1:0.1, 0.75:1:0.1, and 1:1:0.1, respectively. The results are consistent with the work of Sun et al., who reported that increasing the molar ratio of PEI to AgNO3 leads to a decrease in the particle size for the reduction of Ag+ ions by linear polyethyleneimine (LPEI).15 Surprisingly, the UV-visible spectra (Figure 3) showed a red shift of the absorbance peak from 402 to 420 nm as the nanoparticle size decreased as a result of changing the BPEI:AgNO3:HEPES molar ratio. This trend is in contrast with general expectation of blue shift with decreasing particle dimension in a medium of constant dielectric properties.24-26 Our seemingly opposite response probably stems from the fact that there is a larger amount of adsorbed BPEI on the Ag [+] nanoparticle surface when the molar ratio of BPEI to AgNO3 increased. The high sensitivity of the position of the surface plasmon bands to the dielectric properties of the surrounding medium is well-known.27 Red shift of the plasmon peak of silver nanoparticles with an increase in the amount of polymer deposited on the nanoparticle surface has been recently reported by Caruso et al.28 Apart from the ratio of the reagents, the size of the resultant Ag [+] nanoparticles could be also controlled by the BPEI molecular weight. Ag [+] nanoparticles synthesized using BPEI of different molecular weights, 1200, 10 000, and 70 000 g/mol, at the BPEI:AgNO3:HEPES molar ratio of 1:1:0.1 showed an increase in the average particle size with increasing BPEI
Figure 3. UV-visible absorption spectra of Ag [+] in solution mixtures of BPEI:AgNO3:HEPES with molar ratios of (a) 0.5:1:0.1, (b) 0.75:1:0.1, and (c) 1:1:0.1, and (d-f) corresponding SEM images of Ag [+] immobilized on oxidized silicon substrates at pH 5 for 4 h.
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Figure 4. FTIR spectra of (a) BPEI solution (20 mL of 1.0 mg/mL BPEI in water was freeze-dried and mixed with KBr); (b) BPEI solution after UV irradiation (20 mL of 1.0 mg/mL BPEI in water was irradiated by UV for 2 h and then freeze-dried and mixed with KBr).
molecular weight (data not shown). A similar effect has also been observed by Shin et al.,29 who used different molecular weights of polyvinylpyrrolidone in silver nanoparticle synthesis. The correlation between the particle size and the molecular weight of BPEI results from an increasing amount of Ag+ ions associated within larger polymer chains. In the case of the BPEI/AgNO3/ HEPES system, coordination of Ag+ to amino groups occurs,30-33 which can be also additionally assisted by the presence of HEPES ions. It is known that HEPES, which contains SO3- groups, is capable of electrostatic bridging of Ag+ to protonated amino groups.34 3.2. Mechanism for Ag [+] Nanoparticle Formation. The use of polyamines10,12 as reducing agents for metal nanoparticle synthesis is well-known. The mechanism of the nanoparticle formation generally suggests oxidation of amino-containing compounds, but detailed studies of the oxidation products are rare. To obtain insights into the mechanism of the UV-assisted Ag [+] nanoparticle formation in BPEI solutions, we have explored whether solutions containing solely BPEI chains undergo any chemical transformations upon exposure to UV light. Panel a in Figure 4 shows the FTIR spectrum of aqueous BPEI solution without UV treatment. Three major vibrational bands were observed in the spectrum: an absorption band at 1308 cm-1 assigned to C-N vibrations of primary amino groups (νC-NH2)35 and CH bending vibrations,22 another absorption band at 1464 cm-1 associated with N-H bending36 and CH2 scissoring2,22,36,37 vibrations, and a third absorption band at 1578 cm-1 due to NH (24) Mie, G. Ann. Phys. 1908, 25, 377-445. (25) Scaffardi, L. B.; Pellegri, N.; Sanctis, O.; Tocho, J. O. Nanotechnology 2005, 16, 158-163. (26) Link, S.; El Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212-4217. (27) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427-3430. (28) Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846-6852. (29) Shin, H. S.; Yang, H. J.; Kim, S. B.; Lee, M. S. J. Colloid Interface Sci. 2004, 274, 89-94. (30) Frattini, A.; Pellegri, N.; Nicastro, D.; De Sanctis, O. Mater. Chem. Phys. 2005, 94, 148-152. (31) Kuo, P. L.; Chen, W. F. J. Phys. Chem. B 2003, 107, 11267-11272. (32) Yi, Y.; Wang, Y.; Liu, H. Carbohydr. Polym. 2003, 53, 425-430. (33) Fu, J.; Ji, A.; Fan, D.; Shen, J. J. Biomed. Mater. Res. Part A 2006, 79, 665-674. (34) Kim, J. H.; Min, B. R.; Kim, C. K.; Won, J.; Kang, Y. S. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 1813-1820. (35) Suh, J. S.; Moskovits, M. J. Am. Chem. Soc. 1986, 108, 4711-4718. (36) York, S. S.; Boesch, S. E.; Wheeler, R. A.; Frech, R. Macromolecules 2003, 36, 7348-7351. (37) Choosakoonkriang, S.; Lobo, B. A.; Koe, G. S.; Koe, J. G.; Middaugh, C. R. J. Pharm. Sci. 2003, 92, 1710-1722.
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vibrations of secondary amino groups.13,22,38 After UV irradiation (panel b in Figure 4), a strong absorption band at 1660 cm-1 appeared, which indicated formation of amide groups.22 Another new feature is the absorption band at 1390 cm-1 which is assigned to νs,COO-. Spectral changes as a result of UV irradiation of the BPEI solutions clearly indicate UV-assisted oxidation of BPEI. The mechanism of thermal and UV oxidative degradation of PEI using FTIR and NMR techniques has been recently systematically explored by Heatley et al.22 The proposed mechanism involves oxidative degradation of BPEI chains, i.e., oxygen-assisted formation of hydroperoxides, followed by chain scission which occurs primarily at the tertiary and secondary amino groups, yielding the formation of formamide and other amide groups as well as the formation of formaldehyde as shown by others,22 which serves as a reducing agent for Ag+ ions in this study. Using FTIR spectroscopy, Haas et al.39 also showed that oxidation of PEI resulted in the formation of considerable amount of secondary amide groups, as well as carboxylic and tertiary amine oxide groups. The FTIR spectra of mixtures of BPEI and HEPES with or without UV irradiation were very similar to spectra of BPEI solutions in water (data shown in the Supporting Information). This shows that the main path of BPEI cleavage remained significantly unaffected in BPEI solutions in the presence or absence of HEPES. The oxidative chain cleavage by UV irradiation of BPEI solutions has been confirmed here by mass spectrometric studies. Figure 5 contrasts MALDI-TOF data obtained from three different samples of 0.1 mg/mL BPEI solutions in water prepared without any treatment (Figure 4a), as well as after exposure to UV for 120 min in an argon (Figure 4b) or air (Figure 4c) environment. The data clearly show that UV-induced scissoring of BPEI chains is oxidative by nature and does not occur in the presence of argon, when no oxygen is present in the BPEI solution. However, mass spectrometry of HEPES (0.1 mM) before and after UV irradiation revealed no UV-induced changes. This result suggests that at a low concentration HEPES only works as a catalyst to promote the reaction instead of working as a reducing agent. FTIR spectra of UV/O2-treated BPEI solutions (Figure 4b) show, however, strong peaks at 1308 and 1464 cm-1, which indicate that a significant amount of primary amino groups remained unoxidized after BPEI chain cleavage. Seeking to confirm that remaining primary amino groups of the cleaved BPEI fragments retain their capability to bind at the surface of Ag nanoparticles, we performed a control experiment, in which BPEI solution before and after UV/O2 irradiation were used for deposition of polymer chains to Ag [-] nanoparticles produced by citrate reduction technique. ζ-potential of Ag [-] nanoparticles as a function of increasing concentration of BPEI chains, with and without UV irradiation, is depicted in Figure 6a. The fact that in both cases potential of initially negatively charged Ag [-] nanoparticles switched its charge to positive upon addition of BPEI solution indicated that a fraction of amino groups was retained in the BPEI chains after UV oxidative chain cleavage. As shown earlier, limiting off of ζ-potential values at increased BPEI concentrations corresponds to saturation of the nanoparticle surface with adsorbed polymer chains and therefore formation of the saturated monolayer.40 Based on the data in Figure 6a, we estimated that the monolayer coverage was achieved at the (38) Srinivasa Rao, P.; Smitha, B.; Sridhar, S.; Krishnaiah, A. Vacuum 2006, 81, 299-306. (39) Haas, H. C.; Schuler, N. W.; MacDonald, R. L. J. Polym. Sci. Part A: Polym. Chem. 1972, 10, 3143-3158. (40) Sukhishvili, S. A.; Chechik, O. S.; Yaroslavov, A. A. J. Colloid Interface Sci. 1996, 178, 42-46.
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Figure 5. Negative ion MALDI-TOF mass spectra recorded from 0.1 mg/mL BPEI solutions (a) in the absence of UV; irradiated by UV for 90 min in (b) argon and (c) air.
Figure 6. (a) ζ potential of Ag [-] nanoparticles mixed with UV treated (∇)/nontreated (0) BPEI as a function of concentration at pH 7.6. (b) ζ potential of Ag [-] nanoparticles mixed with UV treated (∇)/nontreated (0) BPEI at monolayer coverage as a function of pH. Concentration of Ag [-] nanoparticles was 5 × 1012 particles/ mL. The polymer adsorption time was 15 min. Error bars show the standard deviation.
polymer concentrations of ∼4 × 10-3 and ∼6 × 10-4 mg/mL for UV-treated and UV-untreated BPEI, respectively. The fact that approximately 7-fold higher concentration of BPEI solution was required to saturate the monolayer after such BPEI solutions were treated with UV indicates the consumption of amine groups during the exposure of BPEI to UV irradiation. Also note that the saturation value of the silver nanoparticle ζ-potential was less positive when UV-treated BPEI was used. Figure 6b shows that smaller ζ-potential values for BPEI-coated nanoparticles were obtained in a wide range of pH values in the case of UV-
treated BPEI. This further confirms that a fraction of BPEI amine groups underwent chemical transformation as a result of UV treatment. Note that the formation of silver nanoparticles could also be achieved by reducing AgNO3 with solutions containing solely BPEI or high concentrations of HEPES. In the case of reduction of AgNO3 by only BPEI, the formation of nanoparticles was significantly slower (by ∼6-fold) than that in the presence of HEPES also with appreciable heterogeneity in nanoparticle shape and size distribution. The resultant colloids were not very stable, as evidenced by precipitation of nanoparticles in 24 h. In contrast, colloids of Ag [+] nanoparticles synthesized using a mixture of HEPES and BPEI were stable for weeks. HEPES buffer has also been used as a reducing agent for gold18,19 and silver20,21 nanoparticles. The formation of gold nanoparticles has been reported using HEPES buffer due to the generation of nitrogencentered cationic free radicals from HEPES in the presence of Au(III) at a relatively high concentration of HEPES (0.1 M).18,19 In our previous work, we have shown that silver nanoparticles could be formed on silicon substrates by reduction with 1 mM HEPES solutions at acidic pH.21 In this study, the concentration of HEPES in BPEI/HEPES solutions was even lower, 0.1 mM, and reduction was conducted at neutral to basic pH values. Control experiments showed that HEPES molecules at such low concentration could not produce silver nanoparticles in the pH range of 6-11 without the presence of BPEI. However, simultaneous use of BPEI and HEPES showed clear synergistic effect and allowed fast synthesis of silver nanoparticles. It is probable that HEPES ions have a catalytic function in the reaction, and they also allow electrostatic interactions between the negatively charged sulfonate groups of HEPES and the positively charged BPEI, as well as formation of HEPES/Ag+ ion pairs assisting accumulation and confinement of silver ions within the polymer chains, resulting in faster reduction of silver ions within the polymer coils. Although HEPES ions significantly affected kinetics of nanoparticle growth, the main path of BPEI cleavage remained significantly unaffected in the Ag+-containing BPEI solutions in the presence or absence of HEPES. The sensitivity of XPS to the local chemical environment of an element provides an excellent means of investigating the elemental binding state as well as surface chemical characteristics
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Figure 7. XPS spectra from (a and b) type I and (c and d) type II samples prepared as described in section 2.3. The surface coverage density of the nanoparticles was about 20 particles/µm2 for both samples.
of silver nanoparticles. Such information offers further insights into the mechanism of nanoparticle formation. Shown in Figure 7 are high-resolution C1s and N1s photoelectron spectra obtained from the samples prepared following Schemes 1 and 2 described in the experimental section. The C1s and N1s spectra are both superimposed by two or more spectral features. Note that, given the distinct sample preparation steps, the C1s and N1s spectral features are entirely from, and only from, the immobilized silver nanoparticles for Scheme 1. In contrast, spectral contributions are from the sampling surface area with and without nanoparticle coverage but all coated with BPEI for Scheme 2. Deconvolution analysis suggested the presence of C1s photoelectron lines at 284.9 eV (resulting from the CH2 group in BPEI) and 286.2 eV (originating from the C-N binding) in both sample types. However, there exists a third C1s photoelectron line at 288.1 eV for the type I sample, i.e., Ag [+] nanoparticles immobilized on
oxidized silicon substrate (Figure 7a). This C1s line can be attributed to the carboxyl C of the amide group (OdC*sNs C),41 which constitutes about 12% of the integrated C1s spectral intensity from the sample. This XPS result further confirms the formation of the amide group during the growth of Ag [+] nanoparticles. A third peak component on the left shoulder of the C1s spectrum in Figure 7c for type II substrate also exists at 287.3 eV. This feature can be attributed to CdO from oxidized citrate ions adsorbed on the citrate-reduced silver nanoparticles.42-44 In addition, the N1s photoelectron spectra are also superimposed with spectral features with peaks situated at 399.8 and 400.8 eV. The former is associated with N atoms in the unprotonated amine group and latter with protonated amine group.41 This XPS result also renders support to the ζ-potential measurements described earlier in that a significant amount of amino groups preserved after UV irradiation of BPEI in air.
Scheme 3. Suggested Mechanism for the Synthesis of BPEI/HEPES Reduced Ag [+] Nanoparticles
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attachment of citrate reduced Ag [-] nanoparticles versus BPEI/ HEPES reduced Ag [+] nanoparticles for the detection of 10-8 M (10 ppb) ClO4- and 8.6 × 10-8 M (5 ppb) SCN-, respectively. Figure 8 shows that, along with large spectral background bands, also present are Raman shifts centered around 928 cm-1, characteristic of ClO4- symmetric stretching vibration (Figure 8a), and around 2110 cm-1, characteristic of SCN- stretching vibration (Figure 8b). It is clearly seen that positively charged Ag [+] nanoparticles exhibit superior sensitivity over commonly used negatively charged citrate-reduced Ag [-] nanoparticles for SERS-based anion detection in water. The details about the mechanism of anion binding to Ag [+] nanoparticles and further quantitative treatment of SERS measurements of a series of anions at varying concentrations using substrates immobilized with Ag [+] nanoparticles will be the subject of a subsequent publication. Figure 8. Comparison of SERS sensitivity of substrates with immobilized Ag [+] (top spectra in each panel) and Ag [-] (bottom spectra in each panel) nanoparticles to (a) 10-8 M (10 ppb) ClO4and (b) 8.6 × 10-8 M (5 ppb) SCN- in water. Preparation of the SERS substrates is described in the experimental section of the text. The surface coverage density of the nanoparticles was about 20 particles/µm2 for both substrates.
In this study, therefore, we have shown that UV-assisted reduction in the BPEI:AgNO3:HEPES mixtures results in formation of positively charged Ag [+] nanoparticles in the presence of oxygen due to synergistic effects of BPEI and HEPES. Scheme 3 summarizes the basic mechanism for the production of Ag [+] nanoparticles. Ag [+] nanoparticles are stabilized through adsorption of positively charged BPEI fragments on the particle surface. These BPEI fragments carry positive charge due to the presence of amino groups, and they also contain a significant amount of amide groups. 3.3. SERS Detection of Anions. In this part of our work, it is demonstrated that the Ag [+] nanoparticles can be used for detection of ultra-trace anions in aqueous solutions using their robust SERS features. Figure 8, panels a and b, shows the comparison of SERS activity of two substrates produced by (41) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers: the Scienta ESCA300 database; Wiley and Sons, Ltd.: London, 1992. (42) Ballutaud, D.; Simon, N.; Girard, H.; Rzepka, E.; Bouchet-Fabre, B. Diamond Relat. Mater. 2006, 15, 716-719. (43) Zhang, Y.; Wang, W.; Feng, Q.; Cui, F.; Xu, Y. Mater. Sci. Eng., C 2006, 26, 657-663. (44) Yedji, M.; Ross, G. G. J. Phys. D: Appl. Phys. 2006, 39, 4429-4435.
4. Conclusions We have developed a novel strategy to synthesize positively charged silver nanoparticles via a one-step photoreduction process using BPEI/AgNO3/HEPES solutions in the presence of oxygen. The particle size and distribution can be controlled by the molar ratio between BPEI and silver ions and/or by the molecular weight of the BPEI chains. The synthesis of silver nanoparticles involves oxidative cleavage of BPEI chains, resulting in the formation of BPEI fragments containing both primary amino and amide groups, and the production of formaldehyde. The possible mechanism involves reduction of AgNO3 with formaldehyde rather than a redox reaction between the amino group of the polyamine and Ag+ as suggested in previous studies. The BPEI fragments provide nanoparticles with positive charge when bound at the surface. We believe that this study provides significant new insights into the reduction mechanism of noble metal salts in polyamine solutions. Importantly, Ag [+] nanoparticles demonstrated superior SERS sensitivity over commonly used negatively charged citrate-reduced Ag [-] nanoparticles toward detection of SCNand ClO4- anions in water, and therefore, are promising for SERS-based detection of anions in aqueous solutions. Acknowledgment. This work was supported by NSF under Grant No. ECS-0404002. Supporting Information Available: FTIR spectra of mixtures of BPEI and HEPES with or without UV irradiation. This material is available free of charge via the Internet at http://pubs.acs.org. LA701236V
