Gold nanoparticle response to nitro-compounds probed by cavity ring-down spectroscopy

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Abstract

The surface plasmon resonance (SPR) response of Au nanoparticles to adsorption of NO2, C6H5NO2, and 2,4-dinitrotoluene (DNT) is probed using the cavity ring-down technique. A uniquely sensitive first-layer response is observed for nanoparticles having a mean diameter of 4.5 nm, which appears characteristic of the –NO2 group. Detection limits of 1.2 nmol/L (29 ppb), 7.6 nmol/L (184 ppb), and 0.17 nmol/L (4.1 ppb), are estimated for NO2, C6H5NO2, and DNT, respectively.

Introduction

Gold nanoparticles display unique chemistry [1] and structure [2], which are often probed through changes in the surface plasmon resonance (SPR) [2], [3], [4], [5], [6]. Although typically used as a bulk refractive index sensor, the SPR shows a chemically distinct response to first-monolayer adsorption [3], [4], [5], [6], [7]. With increasing confinement, the influence of specific nanoparticle-surface chemistry or nanoparticle-support interactions [8] becomes more pronounced as the relative fraction of surface atoms increases [9]. Here, we explore the SPR optical response of small Au nanoparticles supported on an ultra-smooth silica substrate to adsorption of NO2-containing species. To probe the SPR, we use cavity ring-down spectroscopy (CRDS) [10], which provides a sensitive and absolute measurement of optical extinction by employing a high-Q optical resonator.

While most studies of NO2 detection by conventional SPR have employed selective films [11], [12], [13], [14], [15], [16], a sensitive and selective response to NO2 also has been observed in experiments on bare Au films [17], [18]. Employing the Kretschmann configuration with a 45 nm thick Au film on the hypotenuse of a glass prism, Ashwell and Roberts [17] found a reversible 0.1% change in the SPR response per 0.1 Pa (1 ppm) of NO2, while reporting a weak or undetectable response for NH3, H2, CO, SO2, HCl, Cl2, and H2S even at comparatively high concentrations. These authors accounted for the unique response to NO2 through the formation of a Au O,O′-nitrito bidentate–chelate complex, which had been observed previously in IR studies of NO2 adsorption on polycrystalline Au films [19]. Although other bonding configurations are also likely to be important [20], [21], [22], the SPR studies on bare Au films suggest a uniquely high sensitivity for NO2 that could extend in a general way to other nitro-containing species. In particular, if an increased degree of analyte nitration results in significantly higher sensitivity, a novel detection mechanism for nitro-based explosives could derive from the Au SPR. For example, 2,4-dinitrotoluene (DNT) is a major impurity and decomposition product in production-grade TNT (1,3,5-trinitrotoluene). Having a higher vapor pressure than TNT, DNT is a prevalent signature species in TNT-based explosives sensing. While CRDS has been employed for direct gas-phase detection of DNT [23] as well as TNT [23], [24] and NO2[25], [26], [27], DNT sensing has also been accomplished by other strategies in which selective coatings were employed, including microcantilever [28], surface acoustic wave [29], and optical [30], [31] techniques, although improvements in sensitivity are needed.

Using CRDS, we recently examined the absolute SPR response of Au nanoparticles to adsorption of chlorinated ethylenes [32], where the intrinsic granularity of an ultra-thin (0.18 nm) Au film was exploited to obtain an approximately Gaussian distribution of nanoparticles having a mean diameter of 4.5 nm. A strong response to adsorption of the chloroethylenes was observed from the Au nanoparticle distribution, yielding detection limits in the ∼10−8 mol/L range for these small molecules. Intrigued by the previous observations of NO2 detection by conventional SPR [17], [18], we explored the SPR response of the previously characterized Au nanoparticle distribution to NO2, C6H5NO2, and DNT, revealing a significantly higher sensitivity for NO2 as well as a trend suggesting this response is generally characteristic of the –NO2 group.

Section snippets

Experimental

The optical configuration for the CRDS measurements is described elsewhere [32], along with a detailed description of the method used to generate and characterize the nanoparticle distribution. Briefly, the CRDS apparatus consisted of a linear optical resonator as shown in the inset of Fig. 1b, incorporating two high-reflectivity mirrors and an intra-cavity flow cell. Ultra-smooth optical flats (0.05 nm RMS roughness; 25 mm dia. × 6.35 mm thick) at Brewster’s angle served as windows to the

Results and discussion

Fig. 1a shows the response of the Au nanoparticle distribution to NO2 for concentrations of 30.1, 41.2, 82.4, and 206 nmol/L (black lines). Uptake is rapid, quickly reaching a stabilized total loss for each concentration. A small gas-phase contribution to the optical loss is incurred due to the path length of the intra-cavity flow cell, corresponding to losses of 12 × 10−6, 16 × 10−6, 33 × 10−6, and 82 × 10−6 for 30.1, 41.2, 82.4, and 206 nmol/L concentrations, respectively, which is comparable to the

Conclusion

In summary, we report a significant improvement (35×) in detection sensitivity for NO2 by using the SPR of small Au nanoparticles, compared to conventional SPR. Moreover, the unique sensitivity to NO2 appears generally applicable to NO2-containing species, increasing with degree of nitration, although a wider range of compounds should be examined to fully elucidate this trend. Furthermore, CRDS has been demonstrated to be a useful diagnostic tool for studies of supported nanoparticles, enabling

Acknowledgement

We gratefully acknowledge support provided by the Environmental Management Science Program (EMSP) of the Department of Energy (DOE) under Contract No. DE-AI07-97ER62518 (Project #73844).

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