Nanomaterials in the Environment: the Good, the Bad, and the Ugly

Abstract

the surface causes changes in the LSPR, giving rise to shifts in the absorption spectrum. Similar to the case of fl uorescence monitoring, the surface of a noble metal particle may be modifi ed to bind a desired analyte selectively, giving a characteristic color change. The absorbance intensity of the newly formed color is proportional to the concentration of analyte present.Surface plasmon formation gives rise to intense colors such as those seen in ancient stained glass windows [3] . Noble metal nanomaterials are frequently used as colorimetric detectors due to their strong absorbance characteristics in the visible region. SPR ( surface plasmon resonance ) band extinction coeffi cients of 2.7 × 10 8 and 1.5 × 10 10 M − 1 cm − 1 (at 520 nm) have been reported for 13 and 50 nm diameter gold particles, respectively [12] . As mentioned previously, changes to the surface structure shifts the observed wavelength of the LSPR and hence gives rise to different colors. The magnitude of this shift depends the mass of the substituent and so the addition of an analyte alone will typically not induce a detectable SPR shift. Further SPR enhancement, however, may be induced by aggregation of NPs in the presence of a specifi c analyte. If bonding between surface groups and the analyte occurs, NPs move from being free -standing systems to large aggregates. This process also causes notable shifts in the LSPR and produces a distinct and easily identifi able color change [13] .To design an effective sensor, surface groups must be chosen such that they will react selectively with an analyte of choice and with a high binding constant. The functionalization of NPs is typically a simple, one -step process. A single noble metal NP may be surface modifi ed in various ways to detect many different analytes. However, unlike single molecule detectors that each have different chemical and physical properties, NP sensors possess similar optical properties because their optical response arises from a common core material. With this knowledge, noble metal NPs may be used to detect a wide range of analytes, including trace metals, pesticides, and gases by changing only the surface group. The most common NP core employed for colorimetric detection is Au; however, Ag has proven useful in some cases.Au -NPs are typically synthesized by citrate reduction of HAuCl 4 , the size of which can be controlled by the citrate concentration [14] . This synthetic procedure results in citrate " capped " Au -NPs whose size may be tuned between 5 and 20 nm. Larger particles, up to 40 nm, can be realized by adjusting the pH and the citrate to Au ratio [15] . Following synthesis and purifi cation, citrate remains physisorbed (i.e., adhered via electrostatic interactions as opposed to chemical bonds) to the surface, which stabilizes the nanoparticles in aqueous solution. The citrate capping is known to interact with various metal ion species, resulting in aggregationinduced colorimetric response at pH 6.7 [16] . Increasing the pH to 11.2 has allowed selective detect...

Effects of water quality parameters on agglomeration and dissolution of copper oxide nanoparticles (CuO-NPs) using a central composite circumscribed design

“…However, a rough estimate of average primary particle size and shape of the CuONPs in the stock suspension, measured from the TEM images, was 39.5 ± 2.03 nm, and the particles were roughly spherical, which fell within the range of 30-50 nm specified by the manufacturer and agreed with the manufacturer's report. The larger size of CuO-NPs measured by DLS compared to TEM may be attributed to the propensity of CuO-NPs to agglomerate in Milli-Q water or the measurement bias of the DLS technique toward large particles or aggregates, which in turn results in increased scattering (Clark et al, 2010;Li et al, 2012;Zhang et al, 2011).…”

Interactions between suspension characteristics and physicochemical properties of silver and copper oxide nanoparticles: A case study for optimizing nanoparticle stock suspensions using a central composite design

“…The larger hydrodynamic diameters measured by DLS techniques might be attributed to the formation of particle agglomerates or aggregates in water, resulting in the increased scattering ability of agglomerates or aggregates in suspension (Clark et al, 2010;Zhang et al, 2011). Moreover, the discrepancies in the primary particle size between stock suspension preparation methods for both nanoparticles may be attributed to the coalescence of neighboring nanoparticles induced by the electron beam as well as the aggregation of nanoparticles while drying/evaporating a sample onto a TEM grid rather than to the difference in stock suspension preparation methods (Murdock et al, 2008;Clark et al, 2010), suggesting that characterization by DLS is more relevant because TEM is not necessarily truly representative of NP arrangement in suspension.…”