Dependence of the LSPR spectrum on the internal properties of NPs (shape, size,
type of material) as well as on the external environment of NPs (Kelly et al. 2003),
make noble metal NPs extremely important for designing optical nanosensors
(Bogue 2004). Detection is based on attachment of target molecule on the surface
of NPs, producing change in the local refractive index, and thus resulting in the shift
in LSPR spectrum. A wide array of molecular recognition elements like DNA,
antibodies or enzymes can be incorporated as self-assembled monolayers (SAMs),
which can be used for chemical modification of NPs and enhance their selectivity.
A portable, cost-effective and simple instrument for transmission mode UV-vis
extinction spectroscopy can be used to implement LSPR nanosensors. The device
consists of a white light source and a small spectrometer, NPs arrays inside a flow
cell and coupling these components using an optical fibre. The target analyte is
stored in a solvent reservoir, and this reservoir and a syringe are also connected to the
cell in the LSPR nanosensors setup. LSPR nanosensors based on single NPs have
also been designed (Haes and Van Duyne 2004).
As the absorbance of individual noble metal NPs is closer to the limit of detection,
LSPR spectrum of single NPs can’t be measured by using UV-visible spectroscopy.
Instead, the LSPR spectra of single NPs can be measured using resonant Rayleigh
scattering spectroscopy, which offers detection of scattered signal in very low
background (McFarland and Van Duyne 2003). LSPR nanosensors can be used
for the detection of analytes at very low concentrations as low as zeptomole
(10
À21 mols) sensitivity and very low volume of analyte (attolitres: 10
À18 L). For
instance, McFarland et al. detected the molecules of hexadecanthiol at zeptomolar
levels. LSPR single-NPs nanosensors provide potential sensing platforms for
multianalyte (McFarland and Van Duyne 2003). Biotin functionalized gold NPs
were used as single-NPs optical sensor for detection of protein streptavidin by
Raschke et al., and this sensor showed sensitive detection for detecting about
50 bound streptavidin molecules (Raschke et al. 2003). LSPR-based nanobiosensors
prepared by NPs functionalized with antibodies have shown sensitive detection
towards microbial toxins and these could be used for successful sensing of water
contaminating microorganisms (Rodrígues-Mozaz et al. 2004).
Colorimetric Nanosensors Noble metal NPs have also been used for colorimetric
sensing applications due to their unique optical properties depending on the particle
size. For e.g., Liu and Lu, reported colorimetric detection of Pb
2+ using gold NPs
functionalized with enzyme specific for Pb
2+ . The sensing is based on aggregation of
gold NPs, displaying blue colour, and change in colour to red with the presence of
Pb
2+ due to cleavage of the substrate by the enzyme and inhibition of aggregation
(Liu and Lu 2004).
Surface-Enhanced Raman Scattering (SERS) As a result of LSPR excitation
displayed by metallic NPs, there is a generation of electromagnetic field on the NPs
surface, which can result in increase in the spontaneous Raman scattering of species
at close distances to the surfaces (Rodriguez-Lorenzo and Alvarez-Puebla 2014).
This phenomenon gives rise to SERS spectroscopy, which can be used as an
100
U. Chakraborty et al.
type of material) as well as on the external environment of NPs (Kelly et al. 2003),
make noble metal NPs extremely important for designing optical nanosensors
(Bogue 2004). Detection is based on attachment of target molecule on the surface
of NPs, producing change in the local refractive index, and thus resulting in the shift
in LSPR spectrum. A wide array of molecular recognition elements like DNA,
antibodies or enzymes can be incorporated as self-assembled monolayers (SAMs),
which can be used for chemical modification of NPs and enhance their selectivity.
A portable, cost-effective and simple instrument for transmission mode UV-vis
extinction spectroscopy can be used to implement LSPR nanosensors. The device
consists of a white light source and a small spectrometer, NPs arrays inside a flow
cell and coupling these components using an optical fibre. The target analyte is
stored in a solvent reservoir, and this reservoir and a syringe are also connected to the
cell in the LSPR nanosensors setup. LSPR nanosensors based on single NPs have
also been designed (Haes and Van Duyne 2004).
As the absorbance of individual noble metal NPs is closer to the limit of detection,
LSPR spectrum of single NPs can’t be measured by using UV-visible spectroscopy.
Instead, the LSPR spectra of single NPs can be measured using resonant Rayleigh
scattering spectroscopy, which offers detection of scattered signal in very low
background (McFarland and Van Duyne 2003). LSPR nanosensors can be used
for the detection of analytes at very low concentrations as low as zeptomole
(10
À21 mols) sensitivity and very low volume of analyte (attolitres: 10
À18 L). For
instance, McFarland et al. detected the molecules of hexadecanthiol at zeptomolar
levels. LSPR single-NPs nanosensors provide potential sensing platforms for
multianalyte (McFarland and Van Duyne 2003). Biotin functionalized gold NPs
were used as single-NPs optical sensor for detection of protein streptavidin by
Raschke et al., and this sensor showed sensitive detection for detecting about
50 bound streptavidin molecules (Raschke et al. 2003). LSPR-based nanobiosensors
prepared by NPs functionalized with antibodies have shown sensitive detection
towards microbial toxins and these could be used for successful sensing of water
contaminating microorganisms (Rodrígues-Mozaz et al. 2004).
Colorimetric Nanosensors Noble metal NPs have also been used for colorimetric
sensing applications due to their unique optical properties depending on the particle
size. For e.g., Liu and Lu, reported colorimetric detection of Pb
2+ using gold NPs
functionalized with enzyme specific for Pb
2+ . The sensing is based on aggregation of
gold NPs, displaying blue colour, and change in colour to red with the presence of
Pb
2+ due to cleavage of the substrate by the enzyme and inhibition of aggregation
(Liu and Lu 2004).
Surface-Enhanced Raman Scattering (SERS) As a result of LSPR excitation
displayed by metallic NPs, there is a generation of electromagnetic field on the NPs
surface, which can result in increase in the spontaneous Raman scattering of species
at close distances to the surfaces (Rodriguez-Lorenzo and Alvarez-Puebla 2014).
This phenomenon gives rise to SERS spectroscopy, which can be used as an
100
U. Chakraborty et al.