Silver nanoparticles for separation and preconcentration processes

Accepted Manuscript Title: Silver nanoparticles for separation and preconcentration processes Author: Kamran Dastafkan, Mostafa Khajeh, Mansour Ghaffari-Moghaddam, Mousa Bohlooli PII: DOI: Reference:

S0165-9936(14)00237-4 http://dx.doi.org/doi: 10.1016/j.trac.2014.08.017 TRAC 14342

To appear in:

Trends in Analytical Chemistry

Please cite this article as: Kamran Dastafkan, Mostafa Khajeh, Mansour Ghaffari-Moghaddam, Mousa Bohlooli, Silver nanoparticles for separation and preconcentration processes, Trends in Analytical Chemistry (2014), http://dx.doi.org/doi: 10.1016/j.trac.2014.08.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Silver nanoparticles for separation and preconcentration processes Kamran Dastafkana, Mostafa Khajeha, *, Mansour Ghaffari-Moghaddama, Mousa Bohloolib a

Department of Chemistry, Faculty of Science, University of Zabol, Zabol, Iran Department of Biology, Faculty of Science, University of Zabol, Zabol, Iran

b

HIGHLIGHTS  Silver nanoparticles (AgNPs) are important in analytical applications  AgNPs are used as nanoadsorbents in preconcentration and clean-up processes  AgNPs can be applied to remove many organic, gaseous and trace-metal species ABSTRACT There is a great deal of interest in the application of nanomaterials, nanotechnology and nanocrystalline inorganics, such as silver nanoparticles (AgNPs), whose intriguing physical and chemical properties have been topics of many scientific research projects. Notable for their extremely small size, AgNPs have been seen in wide-ranging fields, such as optical probes, optical sensors, and plasmonic and biomedical applications. Since AgNPs show different properties, such as surface-plasmon resonance, large surface area, catalysis, and quantum size effects, they can be used for bioassays based on electrochemistry, silver-enhanced fluorescence, surface-enhanced Raman scattering, colorimetry and chemiluminescence. In this review, we highlight the potential of AgNPs as nanoadsorbents for the analytical processes of extraction, separation and sample pretreatment. Keywords: AgNP Clean-up Extraction Nanoadsorbent Nanotechnology Preconcentration Sample pretreatment Separation Silver nanoparticle Trace metal Corresponding author. Fax: +98 542-2226765. E-mail address: [email protected] (M. Khajeh)

1. Introduction Nanoscience and nanotechnology offer new opportunities for making superior materials for use in industrial, health, and environmental applications [1–5]. Nanoparticles (NPs), the primary 1 Page 1 of 15

building blocks of many nanomaterials, are of particular interest in various studies, as the fate of NPs in aqueous environments will depend to their extraordinary properties and widespread range of applications in different scientific and industrial backgrounds. NPs (i.e., particles with structures ~1–100 nm in size) have a significant impact in many scientific fields, including chemistry, electronics, medicine, biology, and material sciences [6–15]. The physical, material, and chemical properties of NPs are directly related to their intrinsic composition, apparent size, and extrinsic surface structure [16–21], so the design, the synthesis, the characterization, and the applications of nanostructures are critical aspects of the emerging field of nanomaterials. One of their important properties is that most of the atoms that have high chemical activity and adsorption capacity for many metal ions are on the surface of nanomaterials [22,23]. The surface atoms are unsaturated, so they are subject to combination with the ions of other elements by static electricity [24], so nanomaterials can strongly adsorb many substances, including trace metals [25] and polar organic compounds [26]. Sample-pretreatment methods, such as separation and/or preconcentration prior to instrumental detection, have developed rapidly due to the increasing need for accurate, precise measurements at extremely low levels of analytes in diverse matrices. Sample-preparation processes, including separation and preconcentration, have a direct impact on accuracy, precision and limits of detection (LODs) of many analytical methods. The concept of NP-assisted sample separation and preconcentration plays important roles in many analytical methods. While primarily associated with increasing the concentration of analytes, the approach provides a number of benefits, ranging from the removal of interfering species to potentially beneficial changes in matrix composition [27]. The potential of NPs was extensively studied in separation science in recent years, and many advances were achieved. Recently, NPs were used as a sorbent due to their intrinsic properties, such as chemical activity and fine grain size, being better than those of classical substances, including normalscale titanium dioxide and alumina [28,29]. NPs were chemically modified by a reagent to obtain a new selective solid-phase extractant for the preconcentration of metal ions [30–32]. As novel materials, NPs play a critical role in removal of environmental pollution as adsorbents, several separation and preconcentration methodologies, which utilize NPs as solid phases, such as solid-phase extraction (SPE), solid-phase microextraction (SPME), µ-solid-phase extraction (µSPE). NPs are also used in chromatographic techniques, including gas chromatography (GC) and high-performance liquid chromatography (HPLC) with columns covered with NPs as stationary phases. These uses of NPs are very important in environmental clean-up. The aim of this article is to review different applications of AgNPs, used as adsorbents for environmental, water, biological and other samples in the past two decades.

2. Silver nanoparticles as adsorbents in separation and preconcentration As mentioned above, despite the wide-ranging applications of AgNPs in different fields of science and technology, the aim of this article is to review those that have been used for analytical purposes and investigated the role of AgNPs as adsorbents in preconcentration and clean-up processes in the past decade. Accordingly, we proceed to the effects of AgNPs in analytical preconcentration and removal of some organic, trace-metal and gaseous species from aqueous, biological and environmental samples.

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2.1. Organic species 2.1.1. Preconcentration of pyrene Olenin et al. [33,34] used adsorption-luminescence methods to combine the preconcentration of polyaromatic hydrocarbons (PAHs), such as pyrene, on the adsorbent surface and their subsequent luminescence determination directly in the adsorbent matrix. For this purpose they used hydrophobic AgNPs with an average size of 4 nm, which were synthesized in two-phase water-organic emulsions. Through the characterization procedure by the instruments, such as molecular spectroscopy and electron microscopy, the physical properties of these AgNPs were evaluated and it was found that they had a specific surface area of 60–110 m2/g. The adsorption-luminescence analysis of PAHs has been an issue of great interest. For this purpose, nanosized organized media, such as AgNPs, deserve special attention in order to improve the accuracy of predicting the conditions of quantitative extraction, and the sensitivity and the specificity of determination in dilute solutions. In this work, CTAB was a surfactant and phase transfer catalyst. CTAB molecules stabilized the surface of AgNPs and transferred them into the organic phase in order to form the organosols of AgNPs. Surfactants are most frequently used in the synthesis of AgNPs for their stabilization, which modifies not only the properties of the test solution, but also those of the adsorbent surface, so the use of CTAB-stabilized AgNPs as adsorbents of pyrene during its determination by the adsorption-luminescence method can radically change the adsorption of pyrene and optimize the spectral parameters of the analytical signal. Eventually, Olenin et al. tested the preconcentration procedures in both organic (nhexane) and aqueous (water) solutions and showed that the resulting AgNPs could successfully serve as adsorbent and preconcentrate pyrene molecules from both dilute (10-8–10-6 g/mL) and concentrated (10-4 g/mL) n-hexane solutions. 2.1.2. Analysis of olefins Sherrod et al. [35] utilized AgNPs in a laser desorption/ionization (LDI) procedure to show selectively ionization of the olefinic compounds [e.g., cholesterol, and 1-palmitoyl-2-oleoyl-snglycero-3-hosphocholine (POPC)] and carotenoids, and finally to analyze them by mass spectrometry (MS). This started with complex mixtures without addition of an organic matrix, sample clean-up, or prefractionation. LDI using NPs differs from matrix-assisted laser desorption/ionization (MALDI) in several ways: (1) less complicated mass spectra in the low mass region due to a decrease in matrix-derived chemical noise (matrix ions and adducts); (2) flexible, relatively simple sample-preparation conditions; (3) higher tolerance to specific chemical additives, such as those commonly used in biological analysis (e.g., surfactants); (4) the ability to tailor the chemical properties of NPs using relatively simple derivatization schemes, which can be exploited; and, (5) modify NPs (metal and silicon based) to capture and ionize analytes selectively on the basis of specific chemical properties (i.e., functional groups). The ability to control size, composition, and electronic properties of AgNPs provides a basis for this procedure. This effect was attributed to π-cation interactions between olefinic compounds and AgNPs. Sherrod and co-workers exploited the silver-olefin interaction to ionize selectively specific carotenoids, a sterol, and a lipid from complex mixtures in the presence of AgNPs without 3 Page 3 of 15

additional washing or extraction procedures. The silver-olefin interaction has long been used for chemical analysis. For example, silver-olefin interactions enhance silver cationization by electrospray ionization [36–39], facilitate olefin transport in supported membranes [40], and separate and quantitate lipids, fatty acids, and triacylglyecerols. [41–44] Moreover, thin silver films have been used to image cholesterol in thin tissue sections of rat kidneys [45] and polymorphonuclear leucocytes [46,47]. Fig. 1 illustrates the selectivity of the process for ionization of olefinic compounds. The priority of nanosized silver is apparent, as shown in Fig. 1b, in obvious contrast with silvernitrate solution (Fig. 1a) [35]. 2.1.3. Removal of Methylene Blue Khajeh et al. [48] developed a simple, fast method for preconcentration and determination of trace amounts of Methylene Blue (MB) from water samples by an AgNP-based solid-phase extraction (SPE) method and ultraviolet-visible (UV-VIS) spectrophotometry. Response-surface methodology (RSM) and a hybrid of artificial neural network- particle swarm optimization (ANN-PSO) were used to develop predictive models for simulation and optimization of the SPE method. The preparation of AgNPs was upon simple chemical reduction of Ag+ ions with silver nitrate and sodium citrate as silver precursor and reducing agent, respectively. The SPE procedure was performed using a plastic syringe with filter as a cartridge filled with different amount of dried synthesized AgNPs. A filter was placed on top to avoid disturbing the adsorbent during passage of the sample solution. The cartridge was treated and washed with methanol and deionized water, then a portion of aqueous sample solution containing MB with adjusted pH was passed through the cartridge. Subsequently, MB retained on the AgNPs was eluted with acetonitrile as eluent. The eluent was analyzed for the determination of MB concentration. The effects of various parameters were investigated, including pH, amount of AgNPs, sample and eluent volume, and sample and eluent flow rates. Adsorption equilibrium studies were conducted at optimum pH with a constant amount of AgNPs and varying initial concentration of MB. The amount of adsorption was calculated based on the difference of MB concentration in sample solution before and after adsorption, according to this formula: Adsorption capacity

(1)

where C0 is the initial MB concentration, Ce the equilibrium concentration of MB in the solution, m the mass of the AgNPs (g) and V the volume of the solution (L). The equilibrium data were analyzed by the Langmuir and Freundlich isotherms and the results exhibited the adsorption capacity of 50.0 mg g-1. Finally, the hybrid ANN-PSO approach was shown to be superior to RSM and was successfully used to study the modeling and the interaction of the variables for the maximum extraction percentage of MB using the experimental data based upon the Box-Behnken design. The procedure was used to separate and to determine the concentration of MB in water samples. 2.2. Gaseous species 2.2.1. Interactions with NO and SO2 Patakfalvi et al. [49] synthesized stable and catalytically-reactive AgNPs with 4.4-nm average size through spontaneous reduction of silver salts in DMF and DMSO at room temperature and 4 Page 4 of 15

used them to interact with pollutant atmospheric gases (NO and SO2). UV-VIS spectra showed the oxidation of silver, the very efficient reduction of NO at room temperature and the adsorption of SO2 molecules on the AgNP surface, causing their aggregation and precipitation. Due to the great solubility of both gases in DMSO, it is possible to carry out direct chemical interaction of AgNPs with NO and SO2 in colloidal dispersions. In order to evaluate the catalytic activity of AgNPs, amounts of NO gas in DMSO were added in proportion to the concentration of AgNPs in solution and UV-VIS spectra and plasmonresonance bands were collected on consecutive additions of NO. Fig. 2 (a and b) shows the electronic absorption spectra of NO gas in DMSO at different concentrations and reacting with silver under anhydrous reaction conditions. The area under the plasmon-resonance band is shown as a function of the amount of NO after every addition (Fig. 2c). The plasmon-resonance band decreases proportionally with each NO addition because the AgNPs immediately react with NO until the silver metal is completely dissolved. The catalytic process of NO degradation on silver surface is also shown in Fig. 2d [49]. The direct interaction between the AgNPs and SO2 solved in DMSO ends in aggregation and precipitation of metallic silver. Fig. 3a shows the absorption spectra of SO2 dissolved in DMSO with different concentrations and reacting with AgNPs (Fig. 3b). In this work, 2-ethylhexanoate and citrate anions were used as stabilizers causing a negative surface electric potential. These anions were displaced by SO2 molecules, so the original electric potential decreased, changing to a more positive value. This change in potential favored aggregation of the AgNPs (Fig. 3c) [49]. 2.3. Trace-metal species 2.3.1. Adsorbent for preconcentration of iron, manganese and lead Khajeh and Dastafkan [50] developed an easy, fast procedure based on SPE intended to preconcentrate, to separate and to determine trace amounts of Fe(ІІІ) ions in biological samples using flame atomic absorption spectrometry (FAAS). AgNPs were synthesized through chemical reduction of Ag+ ions in mixed slurried silica gel-silver nitrate solution by sodium citrate as reducing agent. AgNPs coated with silica gel were modified by 3,5,7,2′,4′-pentahydroxyflavone (morin) and then used as a sorbent. Silica gel as a supporting material is of particular interest because it does not swell and has good mechanical strength, thermal stability, and a large surface area. However, due to its ionexchange silanol groups, silica gel can be directly employed as an adsorbing material to separate and to preconcentrate elements. Furthermore, it is inexpensively available in analyte-free form. However, because of the less pronounced donor properties of the surface oxygen and the low acidity of the silanol group, its interaction with some elements is rather weak. Accordingly, it can be modified with inorganics, such as metal NPs, to enhance its predisposition towards the analytes. In this study, silica gel was first coated in a simple way with AgNPs, and then morin was bound on its surface strictly on the AgNPs immobilized on silica gel to increase the sorption of iron. The effects of experimental conditions were studied, including pH, sample and eluent flow rates, and the type and the least amount of an eluent to the elute iron from the sorbent, and optimum values of these parameters were found. The precision and the accuracy of the adsorption and the separation of Fe(ІІІ) ions were investigated using LOD and relative standard deviation (RSD%). Applying AgNPs as adsorbent measured the Fe(III) concentration in the sample solution directly without a matrix separation step. The sensitive procedure developed was well suited to fast sorption of iron from large volumes of various biological samples. 5 Page 5 of 15

A simple and fast method for preconcentration and determination of trace amounts of manganese [31] and lead [32] in biological samples was developed with modified AgNPs followed by inductively-coupled plasma optical emission spectrometry (ICP-OES) for manganese, and graphite-furnace atomic absorption spectrometry (GF-AAS) for lead. The technical feasibility of AgNPs for manganese removal was investigated in batch studies. Using factorial design and RSM based on the Box-Behnken design, some factors influencing the recovery of manganese were investigated including pH, time and amounts of the complexing agent, and adsorbent. In this work, 1-(2-pyridylazo)-2-naphthol (PAN) was used as the complexing agent for modifying the surface and the stabilization of AgNPs. Thermodynamic parameters indicated the adsorption process was exothermic. The multivariate optimization of the procedure allowed accurate optimum values of experimental parameters to be determined and the possibility to evaluate the interaction between variables with fewer experiments. For the adsorption of lead on the surface of AgNPs, morin was used as a complexing agent. The effects of different parameters were studied and optimized, such as pH of solution, sample flow rate, type, flow rate, and least amount of eluent for elution of the lead from the AgNPs. AgNPs were packed between two filters and placed in a plastic syringe and the whole set was used like a cartridge in the SPE-column technique. The penetration of complexing agent or modifier was allowed completely inside the AgNPs, so the complexing between lead and morin occurred on the NPs. Thus, the lead was retained on the AgNPs, while the solution was passed out of the column. Characterization of nanoadsorbent and the formation of complex with modifying agent were performed by transmission electron microscopy (TEM) and IR. Complexation between AgNPs as adsorbent and modifying agent was evaluated in IR studies. 2.3.2. Preconcentration of mercury Chen et al. [51] showed that AgNPs modified with sodium 2-mercaptoethanesulfonate (mesna) exhibit strong surface-enhanced Raman scattering (SERS). They used mesna as modifying agent for AgNP surfaces to form complexes of mercury(II) ions binding to reactive adsorption sites on the sorbent surface. The covalent bond formed between mercury and sulfur is stronger than that between silver and sulfur, so it prevents the adsorption of mesna on the surface of AgNPs. This results in a decrease in the intensity of SERS in the presence of Hg(II) ions. Several parameters were optimized, including the effects of the concentration of mesna, the concentration of sodium chloride, and incubation time and pH value on SERS. Under optimal conditions, the intensity of SERS decreased with increasing concentration of Hg(II). Fig. 4 shows this process – determination of Hg(II) ions adsorbed on covalently-modified AgNPs with mesna. Mesna molecules can adsorb on the surface of AgNPs using the –SH group, which can enhance SERS signals. The mesna molecules were released from the surface of AgNPs in the presence of Hg(II), which caused a decrease in the observed SERS signal. The experimental results indicated that the SERS signal decreased as the Hg2+ concentration increased. In this way, a quantitative determination for Hg2+ was developed. This method was successfully applied to the determination of the Hg(II) in spiked water samples. 2.4. Quantitative capacity One of the most important and critical parameters that must be considered for every adsorption process is the quantitative capacity of the nanoadsorbent. The adsorption capacity is an important factor because it reflects how much sorbent is required to concentrate the analytes 6 Page 6 of 15

from a given solution [52]. By getting near to the nanometric scale, the porosity of the surface appears advantageous, as do many other features differing from those of bulk materials. The porous surface structure should be considered a factor providing an increase in surface area. These pores decrease the mass-transfer resistance and assist in the diffusion of metal ions because of the high internal surface area with low diffusion resistance in the nanometric sorbents, which ultimately provides high capacity and rate of adsorption [53]. 2.5. Practical considerations in recycling silver nanoparticles Apart from the numerous advantages of utilizing AgNPs for sample-pretreatment applications, prolonged exposure to AgNPs might cause environmental problems, and their recycling is usually difficult. Hence, sources and discharges of AgNPs into various compartments must be quantified and there is a need to develop sample-pretreatment and separation technologies in order to retain, to regenerate and to recycle AgNPs in systems where their functions and reuse are desired. Regeneration and reuse of AgNPs as nanoadsorbent materials along with their recycling becomes increasingly difficult with increasing sorption capacity as the adsorbed compounds need to be released from an intricate network of nano-pores and meso-pores. As other nanoadsorbents, AgNPs cannot be separated easily from aqueous solutions by simple filtration or centrifugation. Centrifugation is one of the most important separation techniques used widely in separation and colloid science [54]. Centrifugal forces can help particles to move radially away from the axis of rotation and thus can separate them. However, while objects denser than a liquid settle spontaneously due to gravity, this process can take a very long time; NPs, where gravitational energy is commensurate with thermal energy, will not settle at all [55]. In order to remedy the poor separation, more powerful separation technologies are needed. An analytical example could be liquid-liquid extraction (LLE). in which chemical adsorption of the solvent onto the surface of NPs is the driving force to separate and to recycle NPs from the intended environment. Extraction is a method to separate compounds based on their relative solubilities in two different, immiscible liquid phases, usually water and an organic solvent. Wilson et al. [56] reported selective extraction of gold and silver dendrimer-encapsulated NPs (DENPs). They synthesized hydroxyl-terminated polymeric dendrimers covered with these NPs and performed the selective extraction of Ag-DENPs (and later Au-DENPs) by adding ndecanoic acid/hexane solution to an aqueous mixture of NPs. After stirring the mixture, the originally colorless organic phase turned yellow, which is characteristic of small AgNPs. The extracted fraction contained 95±6% Ag and 5±6% Au-DENPs. After removing the organic phase, ascorbic acid was added to the aqueous phase (to increase the ionic strength), followed by the extraction of Au-DENPs with n-dodecanethiol/hexane solution. The second extracted fraction contained 8±6% Ag and 92±6% Au-DENPs. Wilson et al. suggested that selective separation of AgNPs based on the LLE method was achievable because n-alkanoic acids (in this case, n-decanoic acid) have higher affinity for chemisorption onto a silver surface rather than a gold surface. Another extraction method for reversible separation/concentration of AgNPs is cloud-point extraction (CPE), which is based on surface-active chemicals, such as NPs, being able to assemble into colloidal-sized clusters called micelles. During formation, these micelles can encapsulate various substances, thus segregating them from the bulk solution. The solubility of non-ionic or zwitterionic surfactants in the water phase is dramatically depressed above a well7 Page 7 of 15

defined temperature called the cloud-point temperature (CPT). Above CPT, a solution separates into a concentrated phase containing most of the surfactant (the surfactant-rich phase) and a dilute aqueous phase. CPE is based on the affinity of compounds/particles of interest toward the surfactant, and this affinity then determines the extent of partitioning between the surfactant-rich and the surfactant-poor phases. Tan et al. [57] extracted and concentrated AgNPs [along with other NPs, such as Au, C60 fullerene, TiO2, Fe3O4, CdSe/ZnS and single-walled carbon nanotubes (SWCNTs)]. They heated the NP solution above the CPT (23–25°C) and added PVP as a capping reagent, Triton X-114 as a surfactant and NaCl to increase the ionic strength of the solution. Due to the NP-micelle interactions, AgNPs were extracted from the aqueous suspension into the surfactant-rich phase and then separated by centrifugation. Also, the concentrated NPs can be redispersed into the aqueous phase by cooling to a temperature below the CPT. Tan et al. proposed at least 10 consecutive separation/dispersion cycles using this method. One of the most promising techniques for analytical separation of NPs in complex matrices is field-flow fractionation (FFF), which is a flow-assisted hydrodynamic separation technique that permits physical separation of small quantities (injected masses in the ng–μg range), macromolecules or particles. FFF is a versatile technique for the separation of various types of NPs (e.g., organic macro-molecules, carbonaceous or inorganic NPs) in different types of media (e.g., natural waters, soil extracts or food samples) due to the availability of different sub-types [e.g., flow-FFF (F4) and SdFFF], a wide variability of run conditions, a relatively broad accessible size range, and possible coupling with sensitive detection systems, such as ICP-MS. The general principles of FFF separation theory and the use of FFF as a separation technique for the study of NPs in complex samples (e.g., food or environmental) were reviewed by Kammer et al. [58]. Meisterjahn et al. [59] applied the F4 method to analysis and separation of AgNPs from soil and sewage-sludge samples. The method was developed by spiking a known amount of wellcharacterized AgNPs to a reference soil and dried sewage-sludge samples from model wastewater-treatment plants. Along with SiO2-NPs and a natural colloidal soil extract (containing a typical assembly of natural NPs found in the environment), AgNP dispersions were used to develop and to evaluate suitable separation conditions for the different samples. The F4 system was coupled to conventional ICP-MS as a sensitive elemental detector. Meisterjahn et al. observed no intensive interaction of the engineered AgNPs with the natural NPs of the colloidal soil extract. Fractograms of colloidal extracts of soil samples spiked with AgNPs showed the presence of free particles and the formation of dissolved silver species, which were probably complexes of silver with humic substances.

3. Silver nanoparticles as analytical probes AgNPs show different physical and chemical properties, such as surface-plasmon resonance (SPR), larger surface area, catalytic properties, and quantum-size effects, which can contribute to the signal amplification of bioassays [60]. AgNP-based assays include detection (e.g., electrochemical, silver-enhanced fluorescence, SERS, colorimetric and chemiluminescent) [61]. There are many published papers in literature on the development of silver labels. For example, Xu et al. [62] reported a method for detection of adenosine based on the change in distance between dye molecules and AgNPs. The method was sensitive and selective. In another research project, α-fetoprotein was detected by using silver nanowire-graphene hybrid nanocomposites as a probe [63]. The method was an easy, sensitive, mediator-free electrochemical immunoassay. 8 Page 8 of 15

An ultrasensitive multiplexed method based on a streptavidin-functionalized AgNPs-enriched CNT as a detection tag was used for carcinoembryonic antigen and α-fetoprotein [64]. Plateletderived growth factor-BB and thrombin were detected using aptamer-functionalized AgNP aggregate developed on an AuNP-modified screen-printed electrode array by differential pulse stripping voltammetry [65]. AgNPs were also a good candidate for colorimetric assay that could be applied for detection of a wide range of biomolecules, such as DNA [66], lectin Concanavalin A and enzymes [67]. Table 1 compares the adsorption capacity of AgNPs and conventional adsorbents.

4. Conclusion and future perspective This review outlined the potential of AgNPs as nanoadsorbents and the many advances that have led to the separation and the preconcentration of a variety of analytes. In the past decade, AgNPs have been a subject of enormous interest. AgNPs, notable for their extremely small size, have the potential for wide-ranging applications. In this review, we explained the applications of AgNPs as adsorbents of several organic, gaseous and trace metal species in sample pretreatment and separation processes. The demands of chemical analysis in modern biology, environmental science, chemistry, medicine, and industry need very high sample throughput and parallel analytical strategies. While the increasing role of AgNPs in separation and preconcentration science is evident, in future, there will need to be greater control over their size and composition. After being utilized for sample pretreatment, AgNPs are difficult to recycle and might cause environment problems, so practical considerations for recycling AgNPs have been addressed. The current interest in the analytical applications of AgNPs is largely related to their high molar absorptivity, their intrinsic capacity to respond to a diversity of chemical environments, physical stimuli leading to a change in their optical properties and numerous sorption sites present on their extensive surface. As a result of recent improvement in technologies for identifying and manipulating AgNPs, this field has seen a huge increase in funding from private enterprise, and academic researchers and government within the field have formed many partnerships [79,80]. References [1] H. Doumanidis, The nanomanufacturing programme at the national science foundation, Nanotechnol. 13 (2002) 248. [2] D.F. Emerich, C.G. Thanos, Nanotechnology and medicine, Expert Opin. Biol. Ther. 3 (2003) 655-663. [3] T. Lowe, The revolution in nanometals, Adv. Mater. Processes 160 (2002) 63-65. [4] K. McAllister, P. Sazani, M. Adam, M.J. Cho, M. Rubinstein, R.J. Samulski, J.M. Desimone, Polymeric nanogels produced via inverse microemulsion polymerization as potential gene and antisense delivery agents, J. Am. Chem. Soc. 124 (2002) 15198-15207. [5] M.R. Wiesner, G.V. Lowry, P. Alvarez, D. Dionysiou, P. Biswas, Assessing the risks of manufactured nanomaterials, Environ. Sci. Technol. 40 (2006) 4336-4345. [6] G. Schmid, M. Bäumle, M. Geerkens, I. Heim, C. Osemann, T. Sawitowski, Current and future applications of nanoclusters, Chem. Soc. Rev. 28 (1999) 179-185.

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Captions Fig. 1. MALDI time-of-flight mass spectra of the 12-component mixture obtained using (a) 2, 5-dihydroxybenzoic acid (DHB) with 120 µM AgNO3 and (b) 20 nm AgNPs. {Reprinted with permission from [35]}. Fig. 2. (a) Electronic absorption spectrum of NO dissolved in anhydrous DMSO; (b) electronic absorption spectra of AgNPs after addition of equal quantities of NO; (c) area under the plasmon-resonance band as a function of the amount of NO in every addition; (d) mechanism of the catalytic process of NO degradation on the Ag surface. {Reprinted with permission from [49]}. Fig. 3. (a) Absorption electronic spectra of SO2 in DMSO at different concentrations; (b) electronic absorption spectra of AgNPs after addition of different quantities of SO 2; (c) mechanism of the interaction between AgNPs and SO2 molecules. {Reprinted with permission from [49]}. Fig. 4. The determination process of mercury(II) ions adsorbed on covalently modified AgNPs with mesna as modifying agent. {Reprinted with permission from [51]}.

Table 1. Comparison of the adsorption capacity of silver nanoparticles with that of conventional adsorbents Detection Adsorbent Analyte Adsorption capacity technique Ref. Modified silica

Hg2+

340–700 mg g-1

-

[68]

Modified silica

Hg2+

200 mg g-1

BHG-AASb

[69]

Functionalized silica gel

Hg2+

3.652 mg g-1

UV-VIS

[70]

Functionalized silica gel IIPa

Hg2+

FISc

[71]

Hg2+

41 mg g-1

CV-AASd

[72]

IIP

Hg2+

25 mg g-1

VGA-AASe, GTA-AASf

[73]

IIP

Hg2+

6.4 mg g-1

CVAAS

[74]

IIP

Hg2+

4.46 mg g-1

ICP-OESg

[75]

C18

Hg2+

0.52 mg g-1

UV-VIS

[76]

Microcrystalline naphthalene

Hg2+

1350 μg g-1

CVAAS

[77]

8 mg g-1

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AgNPs

Hg2+

0.8 g g-1

ICP-OES

[78]

a

Ion-imprinted polymer Borohydride generation-atomic absorption spectrometry c Flow-injection spectrophotometry d Cold-vapor atomic absorption spectrometry e Vapor-generation accessory-atomic absorption spectrometry f Graphite-tube atomizer-atomic absorption spectrometry g Inductively-coupled plasma-optical emission spectrometry b

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