Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
pubs.acs.org/est
Dry Powder Assay Rapidly Detects Metallic Nanoparticles in Water by Measuring Surface Catalytic Reactivity Xiangyu Bi,*,† Hongfang Ma,†,‡ and Paul Westerhoff† †
School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona 85287, United States Department of Municipal Engineering, College of Civil Engineering, Huaqiao University, Xiamen 361021, China
‡
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S Supporting Information *
ABSTRACT: We designed the “catalytic reactivity to nanoparticle” assay (CRNP), which uses a dry powder containing methylene blue (MB) and sodium borohydride (NaBH4) to rapidly (2 min) detect metallic nanoparticles in water. Tested with gold (Au) NPs in water, the CRNP response was linearly and reproducibly correlated to the NP surface-area concentration and has a detection limit of 0.3 m2/m3 as the equivalent surface area of Au NPs. We described the heterogeneous catalytic mechanisms on the NP surface by treating the NPs as electrodes, which store and transfer electrons, and comprehensively simulated the kinetics of borohydride hydrolysis, MB reduction, and leuco methylene blue (LMB) oxidation. CRNP was able to assess the catalytic reactivity of multiple engineered NP species in water, including Au, silver, palladium, platinum, and copper oxide (CuO), and quantify them with pre-established calibration curves. In water samples containing known or unknown NP species, CRNP can be reported as an equivalent surface area of gold NPs per volume of solution and directly quantifies NP reactivity in response to electron mediated stimuli, which may become relevant to the environmental fate or safety of nanomaterials.
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INTRODUCTION Engineered nanoparticles (NPs) are widely used in industrial and commercial products, e.g., catalyst composites, coatings, and medicine,1−3 and their accidental releases into aqueous media in the environment are possible. Tracking NPs in water systems is needed to assess their exposure and risks.4,5 Existing techniques for detecting NPs in natural waters are often costly, time-consuming or both.6 For example, techniques hyphenated with the inductively coupled plasma mass spectroscopy (ICPMS), including single-particle,7 flow-field fractionation,8 and high-pressure liquid chromatography (HPLC),9 have complicated and expensive setups despite their sensitivity in analyzing metallic NPs in water. Transmission electron microscopy (TEM) can characterize NP size and morphology, but it is costly and laborious, and it is difficult to deduce low abundance or concentration in an environmental sample.10,11 Some techniques have thus been employed to extract lowconcentration NPs from liquids (e.g., “cloud-point extraction”) 12,13 prior to analysis. However, the rapid and economical detection of NPs in water was not satisfactorily achieved. Analyses of many environmental pollutants (e.g., chromium, chlorine, organic nitrate, and chemical oxygen demand) are greatly facilitated by commercialized, rapid, and easy-to-use colorimeter-based assays. They are understood to be surrogates or complements for more advanced instruments, and the drawbacks of potential interferences are wellunderstood. Such an assay exploits a predesigned chemical © XXXX American Chemical Society
mixture, typically in powder form, to react with a target compound, resulting in a colorimetric change (e.g., absorbance or fluorescence) that can be measured and related to the target’s concentration. The assay often serves as a reliable screening-level technique to monitor contaminants in environmental wet laboratories or in the field. The same strategy can be applied to specific NP classes. Colorimetric probes based on fluorescence14 or absorbance15 exist to detect metallic NPs, but they are often used in wet-chemical forms and require precise and tedious laboratory handling. This motivated us to consider fundamental assay chemistry, develop a chemical power, and enable an assay kit for rapidly detecting metallic NPs in water. Designing a powder assay kit requires a reaction and thus relies on NP reactivity. Many metallic NPs, e.g., gold (Au), silver (Ag), iron (Fe), palladium (Pd), and platinum (Pt), are catalytic reactive and can invoke accelerated redox reactions on their surface relative to homogeneous reactions occurring in the liquid phase.16−21 Therefore, they are often applied in engineering fields including hydrogen generation,22 fuel cells,23 and pollutants degradation.24 When catalyzing a redox reaction, the NP surface mediates electron transfer from a reductant to an oxidant.25 To the extent that metallic NPs are used to catalyze desired redox reactions, a well-controlled Received: Revised: Accepted: Published: A
July 16, 2018 September 13, 2018 October 17, 2018 October 17, 2018 DOI: 10.1021/acs.est.8b03915 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
samples was performed in a previous study and used in this work.31 A copper oxide (CuO) nanopowder (Sigma-Aldrich, 544868, Pt > Au > Ag > CuO. The selected five metallic (metal or metal-like) NPs demonstrated catalytic activity in specific reactions.48−51 A systematic comparison of the catalytic reactivity of the different metallic materials with the same reaction was not found. We expected CuO would have the lowest surface catalytic reactivity because of being an oxide material, which has lower electron mobility than metals in general. Esumi et al.
Supporting Information and leads to the MB reduction reaction rate described by eq 7: dC MB+ dt
(6)
ÅÄÅ F ÑÉÑ mass = − 2C NP A m k 30expÅÅÅÅ− (E10 + E20 − 2E30)ÑÑÑÑ ÅÅÇ 4RT ÑÑÖ ij k 0 yz × jjjj 0 1 zzzz j k 2 CH O z 2 { k
1/2 1/2 −×C × C BH MB+ 4
(7)
Cmass NP
where and Am are the mass concentration and specific surface area (surface area per mass) of Au NPs; F, R, and T are Faradaic constant (96485 C mol−1), ideal gas constant (8.314 J K−1mol−1), and temperature (K), respectively. Eq 7 predicts that the rate of eq 6 is a first order to MB and a half order to BH4−. The outcome of the half order is based on the assumption that electron-transfer coefficients of reactions 4−6 all have a value of 0.5 (see the Supporting Information), which may slightly deviate from the actual value (0.3−0.7 in most cases).44 A non-integer reaction order less than one with respect to BH−4 from experimental data has also been reported for heterogeneously catalyzed BH−4 hydrolysis.45,46 An overall kinetic model taking account of all reactions (eqs 1 to 3) is therefore readily formulated as in Table 1. Notably, Table 1. Kinetic Reactions and Initial Conditions to Fit Experimental Data reactions
dC BH−4 (t ) dt
= − k1C BH−4 (t )
initial conditions
CMB(0) = C0,MB
(8)
dCMB(t ) = − k 2,obsC MB+(t) + k 3C LMB(t) dt
(9)
dC LMB(t ) = − k 2,obsC MB+(t ) − k 3C LMB(t ) (10) dt 1 1 1 = + k 2,obs k 2,m k 2,e × C BH−(t)1/2 (11)
C LMB(0) = 0 C BH−4 (0) = C0,BH−4
4
because MB reduction is a surface reaction, its observed rate constant, k2,obs, is split, as given by eq 11, into a mass-transfer component (k2,m) and a surface electron-transfer component (k2,e), both of which are proportional to NP surface area in solution.40,44 k2, e corresponds to eqs 6 and 7, with C1/2 BH4− separated out. The kinetics model was numerically solved, with k1, k2,m, k2,e, and k3 fitted using experimental data in Figure 2. The model (dashed lines in Figure 2) agrees well with experimental data except the highest Au NP concentration, where the discrepancy is likely due to the mass-transfer limitation when BH−4 is over-consumed. The experimentally validated model supports the proposed “electron transferring through NP” mechanisms, suggesting that the assay works exclusively for materials with electron mobility (a feature of metallic materials). Values of k1, k2,m, k2,e, and k3 are plotted versus Csurface (m2/ NP 3 m ) in solution (Figure 4). k1 (Figure 4a) remains constant (p < 0.05) among all NP surface area concentrations and averages (0.029 s−1), which is close to the value (0.025 s−1) reported in a previous study.38 k2,m is linearly correlated to Csurface (Figure NP 4c), where the slope (0.014 m/s) denotes the MB masstransfer coefficient in the system (denoted by mMB, with the dimension of [L][T−1]. The mass-transfer coefficient of a reactant to spherical ultramicroelectrodes theoretically equals its diffusion coefficient (DMB) (with the dimension of E
DOI: 10.1021/acs.est.8b03915 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 4. Model fitted parameters: (a) k1, (b) k2,e, (c) k2,m, and (d) k3 versus surface area concentration. Error bars represent two times the standard deviation of triplicate samples. Linear regression with equation y(x) = kx was performed for k2,e and k2,m, and the obtained slopes represent kS2,e and mMB, respectively.
samples is expressed as nanograms per liter of estradiol (E2) equivalents. Similar to the chemical oxygen demand (COD) assay, which expresses conversion of dichromate to chrome caused by unknown organics and inorganics in water as an equivalent concentration of oxygen (milligrams of O2 per liter) needed to stoichiometrically achieve the same conversion, the CRNP can be normalized and expressed in equivalent units. Here, we express the catalytic reactivity probed by the CRNP assay as the equivalent surface area of tannic acid coated Au 3 2 NPs (Csurface Au−eqv, mEQ − AuNP/m ). To illustrate this, we compared the analyses of 150 μg/L Au NPs (50 nm) suspended in ultrapure water (matrix 1), surface water (matrix 2), filtered surface water (matrix 3), and water containing SRNOM in the range of 0.5−10 mg/L as C (matrix 4). Figure 6 presents the outcomes in ΔA663 and in Csurface Au−eqv
Figure 5. Calibration curves (ΔA663 vs Csurface NP ) for Au, Ag, Pd, Pt, and CuO NPs.
demonstrated that, in the reaction of reducing 4-nitrophenol, the catalytic reactivity of different NP materials showed an order of Pd > Pt > Ag, agreeing with our finding.50 Elsewhere, Au NPs were more catalytically reactive than Ag NPs in solution using the reaction of borohydride reducing 4nitrophenol;52 yet another study suggested otherwise when they were attached onto reduced graphene oxide mat to catalyze the same reaction.53 However, neither study clarified the actual surface area in the catalytic reactions, whereas our findings suggested Au has higher surface-normalized catalytic reactivity than Ag. With each calibration curve in Figure 5, the concentration of one NP species can be estimated by measuring ΔA663 using CRNP. Second, CRNP can detect the catalytic reactivity of known or unknown NPs in an environmental water sample. Many assays use equivalent units. In biological systems, estrogenicity associated with potentially numerous specific and often unknown compounds in wastewater or other environmental
Figure 6. Catalytic reactivity of NPs analyzed by the powder assay for 150 μg/L Au NPs (50 nm) suspended in ultrapure water (matrix 1), surface water (matrix 2), filtered surface water (matrix 3), and water containing SRNOM (0.5−10 mg/L as C, matrix 4). Error bars stand for the standard deviation for triplicate measurements. F
DOI: 10.1021/acs.est.8b03915 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology (calibrated by Figure 1b). Compared against matrix 1, matrix 2 showed higher (p < 0.05) catalytic reactivity, suggesting the presence of other catalytically reactive compounds, which are likely also metallic colloids, other than the spiked Au NPs. Matrices 3 and 4 showed statistically insignificant (p > 0.05) variance from matrix 1, suggesting that the reactive compounds in the surface water were effectively removed by the glass fiber filter and the dissolved NOM puts negligible impact on the surface catalytic reactivity of Au NPs. This was further proved by assay tests with Au NPs suspended in water containing different levels of Suwanee River NOM (Figure S8). There are cases where the detected catalytic reactivity is contributed by “unknown” particles, and other techniques are needed to identify the reactive species. For example, we found a certain constituent in tap water with a strong catalytic reactivity by CRNP, and further analysis showed it could be certain species containing copper (Figure S9). CRNP may be able to prescreen NP presence in environmental waters. Because catalytic activity may be an indictor of potential adverse toxicity outcomes,54,55 the assay may be used as a screening test for the environmental health and safety (EHS) of nanomaterials. More research directly correlating CRNP to toxicity would be needed to confirm this premise. CRNP, like many other chemical assays, may bear artifacts when interfering chemicals exist. Insofar as it is too ideal to invent an artifact- and interference-free assay, we recommend careful examination and manipulation of the assay solution chemistry to identify and eliminate an interference. For example, we found that silver ions (Ag+), which can be released from Ag NPs under an oxidizing environment, is also reactive in the CRNP assay, causing artifacts for measuring the surface reactivity of Ag NPs. However, the addition of NaCl at 1 mM (a concentration level commonly found in natural water) can screen the reactivity of Ag+ by the complexation of Cl− and Ag+ and thus eliminate the artifacts. Details are given in the Supporting Information (Figure S10 and thereafter). Other than NaCl, other chemical agents that have strong affinity to metal ions, e.g., ethylenediaminetetraacetic and diethylenetriaminepentaacetic acid (EDTA and DETAPAC), may also eliminate this type of interferences. Overall, the fundamental insight gained by the CRNP assay suggests at least two important applications. First, the assay can be used as a rapid indicator to track known NPs (e.g., Au NPs) throughout reactors (e.g., environmental systems and biota). Second, the assay can be used to detect “unknown” particles in water by their catalytic reactivity, an indicator of the surface catalyzing potential of particles. Thus, the assay may also be useful to increase understanding of how environmental conditions (e.g., sulfidation or NOM) passivate NP surfaces or, as illustrated by the difference between filtered and unfiltered surface water, demonstrate the relative presence of surface catalyzing particles already in our water systems.
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performance, influence of NOM presence on assay outcome, catalytic reactivity and the total Cu concentration of tap water samples, investigation on assay interference posed by Ag + , and a mathematical description of surface reactions (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiangyu Bi: 0000-0003-2706-3039 Paul Westerhoff: 0000-0002-9241-8759 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially funded by the U.S. Environmental Protection Agency through the STAR program (grant no. RD83558001), the NSF Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (grant no. EEC-1449500), and by the NSF (grant no. CBET1507750) as a GOALI project with the Semiconductor Research Corporation (SRC, 425.052). The authors thank Drs. Charlie Corredor, Jonathan Posner, and Pierre Herckes for their insightful suggestions regarding this work.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b03915. TEM images and absorbance spectra of NP samples, deconvolution of the MB absorbance−time spectrum, predicted detection limits, absorbance-wavelength spectra during the assay reaction, observed first-order rate constants, influence of parameter variance on assay G
DOI: 10.1021/acs.est.8b03915 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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