Letter pubs.acs.org/ac
In Situ Colorimetric Quantification of Silver Cations in the Presence of Silver Nanoparticles R. A. González-Fuenzalida,† Y. Moliner-Martínez,*,† María González-Béjar,*,‡ C. Molins-Legua,† J. Verdú-Andres,† Julia Pérez-Prieto,‡ and P. Campins-Falcó† †
Department of Analytical Chemistry, University of Valencia Dr. Moliner 50, 46100 Burjassot, Valencia, Spain Instituto de Ciencia Molecular (ICMol)/ Departamento de Química Orgánica, Universidad de Valencia, C/Catedrático José Beltrán 2, 46980, Paterna, Valencia, Spain
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S Supporting Information *
ABSTRACT: Silver ions (Ag+) can be quantified in situ in the presence of AgNPs by using a colorimetric sensing probe (3,3′,5,5′-tetramethylbenzidine). Interestingly, it also enables detection of the Ag+ adsorbed on the AgNP surface. This is relevant to design new methods to make AgNPs while ensuring the total reduction of Ag+.
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make AgNPs or to improve currently used ones. Moreover, an “in situ” protocol will be developed to follow the reaction progress and determine the real yield of AgNP production as well as to estimate the specific amount of reductant needed for each synthesis.
ilver nanoparticles (AgNPs) are nowadays the most widely commercialized nanomaterial (hundreds of tons per year).1 Their antibacterial properties make them suitable for applications based on consumer-oriented products, such as components of detergents, clothing, fabrics, food additives, and medical devices, among others.2−7 AgNPs progressively release Ag+ ions from their crystalline core under aerobic conditions.8,9 In fact, there is an open debate on which species is actually the toxicant, the AgNPs or the Ag+ ions released from their surface. It has been postulated that the antibacterial activity of Ag+, as well as their environmental impacts, could be controlled by modulating its release from the AgNPs.10 Moreover, studies performed in natural waters under field conditions suggest that the release of Ag+ is highly influenced by the size and capping of the AgNPs.11 The initial and dominant process seems to be desorption of chemisorbed Ag+ from the nanoparticle surface,7 while the oxidative dissolution seems to be considerably slower. On the other hand, the development of simple probes to determine the production yield of AgNPs from Ag+ reduction is also a demand for the industry of nanotechnology. Currently, all methods used to quantify AgNP production require previous conversion of the AgNPs back to Ag+: inductively coupled plasma mass spectrometry (ICPMS) combined with an extraction procedure (cloud-point extraction12,13 or field flow fractionation14−16); size exclusion chromatography;17 counter current chromatography;18,19 electrophoresis;20 and fluorimetry.21 Consequently, the aim of this work was to find a method to be able to differentiate easily between AgNPs and Ag+ “in situ”. This will be highly applicable to develop new methods to © 2013 American Chemical Society
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EXPERIMENTAL SECTION Derivatization Procedure and UV−Visible Measurement. A volume of 1.2 mL of Ag+ standard solutions, AgNP solutions, or AgNP solutions spiked with Ag+ were mixed with 100 μL of TMB solution (10 mM in ethanol) and 200 μL of NaAc/AcH (pH = 4) buffer solution. After the optimum reaction time, the solutions were directly measured with a fiber optic probe of the UV−vis spectrophotometer. Optimization of the TMB concentration and reaction time was carried out. Aliquots from photochemical or thermal syntheses under different conditions were processed with the proposed procedure. AgNPs using thermal synthesis were diluted (1:2) before being processed. AgNPs Purification. Volumes of 1.2 mL of AgNPs solutions were passed through cationic exchange cartridges (Strata SCX, benzene sulfonic acid group bounded to silica particle, 55 μm, 3 mL), previously conditioned with 1 mL of methanol and 1 mL of water. The derivatization reaction was carried out in the eluted solution. Received: September 5, 2013 Accepted: October 16, 2013 Published: October 16, 2013 10013
dx.doi.org/10.1021/ac402822d | Anal. Chem. 2013, 85, 10013−10016
Analytical Chemistry
Letter
Scheme 1. (A) Oxidation of TMB by Ag+; (B) Schematic Representation of the “in Situ” Detection of Residual Ag+ after the Synthesis of AgNPs; (C) in Situ Colorimetric Visualisation, Ag+ (4 ppm)
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RESULTS AND DISCUSSION Recently, Liu et al. have used 3,3′,5,5′-tetramethylbenzidine (TMB) as a selective and sensitive indicator of Ag+ (Scheme 1A).22 The addition of TMB and a buffer solution (NaAc/ AcH) to an aqueous colorless solution containing Ag+ gives rise to the oxidized form of TMB (oxTMB) and Ag(0). This can be followed by absorption spectroscopy (typical peaks of oxTMB at ∼370 and 655 nm) and visually detected due to the color change from yellow to blue (Figure 1, vial 1). We demonstrate
Table 1. Synthetic Strategies Used Here to Make AgNPs and Residual [Ag+] Concentration Obtained with the Colorimetric Probe synthesis strategy photochemical UVA lamp
T (°C) room temperature
thermal (1)
reagent I-2959 citrate
NaBH4 (0.1%) 70
thermal (2) a
95
NaBH4(0.04%) citrate/glycerol
time (min)
[Ag+] (ppm)
5
2.0 ± 0.9a
10 15 20 5 10 15 60 60 60
1.3 1.6 1.5 3.6 2.7 2.7 2.5 36.0
