Highly sensitive ratiometric quantification of cyanide in water with gold nanoparticles via Resonance Rayleigh Scattering
Graphical abstract
Introduction
Cyanide is one of the fastest and most powerful poisons due to its high tendency to attach to cytochrome c oxidase, inhibiting its function in the electron transport chain and leading to cytotoxic hypoxia, cellular asphyxiation and lactate acidosis that finally result in death within minutes without adequate treatment [1].
Despite its well-known toxicity, the use of CN− is widely spread in many industries such as in the synthesis of fibers like nylon, and many other metal and organic chemical factories. Cyanide is also used in the extraction of gold and silver because of its ability to coordinate these highly valued metals enabling their extraction as soluble species. This widespread application turns contamination by CN− into a very severe health and environmental problem in countries such as Peru, where a high percentage of (illegal) mining skips regulatory controls. For all of these reasons, the study of CN− levels in water sources is indispensable for stablishing appropriate controls to avoid public health and environmental disasters.
Several methods have been developed for the detection of CN− [2], [3] but their LODs are often above the maximum acceptable level for drinking water established by WHO (1.9 µmol L−1), need quite highly complex and expensive molecules [4], [5], [6], require special sample preparation or the use of organic solvents [7], require extended analysis times or are based on complex instrumentation [8].
In this context, the unique chemical and physical properties of nanomaterials (e.g. optical, electrochemical, catalytic) have been exploited in the design and application of chemical sensors for proteins, DNA, metallic ions and small molecules. Nanomaterial-based chemical sensors offer distinct advantages including high sensitivity and selectivity, rapid detection times and low cost [9], [10], [11]. AuNPs are among the most frequently employed nanomaterials for this purpose because they can be prepared by simple and reproducible synthesis methods and can be easily functionalized with a variety of molecules [12], [13].
AuNPs are widely used as optical sensors due to their high extinction (absorption plus scattering) coefficients within their localized surface plasmon resonance (LSPR) band, which falls in the visible electromagnetic region [14]. Most frequently, the colorimetric (extinction) response of these sensors is monitored using conventional UV–Vis spectrophotometers or even by the naked eye [13], [15], [16].
Some AuNPs-based chemical sensors for CN− determination have been reported. These are based on the ability of CN− to etch metals, which leads to observable changes in the LSPR band or to a signal from a reporter molecule [17], [18]. Liu et al. described a colorimetric sensor for CN− using 13 nm spherical PS-40 functionalized AuNPs with a LOD of 500 nmol L−1 [19]. In this report, CN− etched the AuNPs to form Au(CN)2− complexes, induced detachment of PS-40 (which acted as a stabilizer) from the surface and consequently caused the aggregation of AuNPs in a high ionic strength medium. Aggregation of AuNPs resulted in a red colorimetric shift due to interparticle plasmonic coupling [20] and thus could be easily monitored by UV–Vis spectroscopy.
RRS is a particular scattering process produced when the wavelength of the incident light is the same or near to an absorption band [21]. Although extremely weak for molecular non-aggregated species, RRS cross-sections of metallic colloids within the LSPR band are sufficiently strong to be exploited in the design of chemical sensors [22] and single AuNP detection assays [23], [24]. For 15 nm-AuNPs, RRS efficiency is comparable to that of fluorescence in terms of light intensity per optical density unit [22]. Furthermore, the RRS component of the extinction coefficient increases relative to the absorption counterpart as the nanoparticle size grows.
Recently, Cai et al. reported the use of a AuNPs-based RRS sensor for folate in living cells with very promising results [25]. This type of sensor has also proven to be very useful for the determination of proteins [26], [27], heavy metals [28] and small molecules [29].
Based on the great potential of AuNP-based RRS sensors and building on the method reported by Liu et al. [19], here we propose a highly sensitive and selective CN− sensor that can be monitored using compact portable instrumentation and even by the naked eye. To our knowledge, this is the first demonstration of a AuNP-based RRS sensor for CN−.
Section snippets
Materials
Chloroauric acid (HAuCl4·3H2O) and PS-40 (Tween® 40) were purchased from Sigma-Aldrich. Sodium nitrate, strontium chloride hexahydrate, sodium fluoride and calcium chloride dihydrate were purchased from Merck. Sodium citrate, sodium sulfate, sodium carbonate, potassium chloride, magnesium chloride hexahydrate, iron sulfate heptahydrate, iron(III) chloride, sodium hydrogencarbonate, sodium nitrite and the rest of reagents were purchased from JT Baker. Milli-Q grade water was obtained with a
Synthesis of AuNPs
We sought to study the influence of the size of AuNPs in the performance of the CN− sensor under both extinction and RRS detection schemes. Two factors were expected to play a central role in the performance of the sensor: colloidal stability and the relative contribution of the absorption and scattering components to the extinction coefficient of the different AuNPs. It is well known that as AuNP size increases, stability and monodispersity usually go down [32]. However, the scattering
Conclusion
Relevant experimental parameters of the CN− nanosensor were optimized for three different AuNP mean diameters: 14 nm, 40 nm and 80 nm. A compromise between stability and scattering efficiency was reached using the 40 nm-AuNP sensor, for which a 10-fold sensitivity enhancement was observed (LOD 0.1 µmol L−1) relative to the 14 nm-AuNP sensor (LOD 1.0 µmol L−1) using both the colorimetric and RRS detection schemes. The sensor is sensitive enough to allow detection of 0.5 µmol L−1 cyanide by the naked eye,
Conflict of interest
The authors declare that they have no competing interests.
Acknowledgments
The authors gratefully acknowledge financial support from Programa Nacional de Innovación para la Competitividad y Productividad, Innóvate-Perú (Grant No. 119-PNICP-PIAP-2015), Dirección de Gestión de la Investigación at PUCP (Grant No. DGI-2015-178), the Chemistry Section at PUCP and Fondo Social de la DGA (grupos DGA). Y.C. thanks Dr. Javier Nakamatsu (Chemistry Section, PUCP) for fruitful discussions and Rafael Coello (Physics Section, PUCP) for advice with the LEDs.
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2022, Spectrochimica Acta - Part A: Molecular and Biomolecular SpectroscopyCitation Excerpt :Investigation of the morphology of Ag NCs by SEM and TEM reinforces the results of the extinction spectrum and shows that Ag NCs with a uniform size of about 42 nm and high purity have been successfully synthesized (Fig. 1b, 1c). It has previously been demonstrated that the scattering cross-section improves with increasing the nanoparticle size [53,54], but this increase in size reduces the nanoparticles' stability [55]. As a result of balancing these two properties, 42 nm Ag NCs were selected as an RRS probe because they exhibit both strong scattering cross-section and good stability.
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2018, Sensors and Actuators, B: ChemicalCitation Excerpt :Nevertheless, due to narrow dynamic range, low sensitivity and especially complicated experimental setup and high cost of apparatus, the practical applications have been limited. Resonance Rayleigh scattering (RRS), which can be conducted with a general fluorescence spectrometer, has been known for its sensitivity, rapidity and simplicity [16]. The technique was first applied to analytical chemistry in 1993 [17], and had been extensively used for the detection of medicine [18], bio-macromolecular [19], inorganic ions [20], surfactants [21] and some physicochemical parameters [22].
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