请为以下文章生成一篇简短的英文演讲稿，内容不超过500词。4.4. Label-Free Optical Detection Based onChanges in Refractive Index Plasmons are exceptionall sensitive reporters for chemical phenomena that influence the refractive index of the local environment of a probe. SPR spectroscopy and imaging are well-known label-free optical detection methods that use this property to monitor surface binding events in real time (as discussed in a separate contribution in this issue). In these techniques, changes in the dielectric environment at the surface of a flat or periodically structured noble-metal film shift the observed SPR resonance, which can be measured using angular interrogation, wavelength interrogation, or intensity measure-ments. This well-established technique enables determination of kinetic and thermodynamic data for a wide variety of molecular binding events, especially those involving bio-molecular targets.371,374,375 Progress is currently being made to develop nanostructured plasmonic materials for performing similar analyses. Changes in the dielectric environment of nanostructured metals, such as NPs, results in measurable shifts of the LSPR peak position and/or magnitude that can be used to perform label-free chemical or biosensing in real time. A varietv of noble-metal nanostructures such as NPs in solution or immobilized on surfaces, nanoholes and nanohole arrays, and nanoisland films379-381 have been used in this way. Substrate-bound nanostructures offer several attractive features as platforms for chemical sensing. These include the shape, size, composition, and spacing of the NPs can be readily controlled to provide tunable peak positions and widths and (ii) the NPs are free of the capping agents or stabilizers used in solution-phase NP synthesis, making their surfaces readily accessible for functionalization with specific receptors or ligands. In the following discussion we will provide some examples of chemical and biosensing based on refractive index changes near the surface of nanostructured metals in solution and on substrates, where the LSPR is monitored using extinction or scattering spectroscopy. The refractive index sensitivity of each sensing platform will also be given where validated data is available. This sensitivity is commonly defined in terms of the change in an experimentally measurable parameter (typically peak position or magnitude per 'refractive index unit' (RIU), which corresponds to a change of 1 in the refractive index. These measurements are usually performed by taking spectra of a plasmonic nanostructure in solutions of increasing refractive index while monitoring peak position or intensity changes. 4.4.1. Nanoparticle Dispersions Au nanostructures are known to exhibit strong plasmonic bands that are dependent on their shape, size, and surrounding media. Ghosh et al.26 studied the effects of changing solvents and ligands on the LSPR of Au NPs dispersed in solution. It was found that the surface plasmon absorption maximum of the Au NPs varied between 520 and 550 nm, depending on the refractive index and chemical nature of the surrounding solvent. The authors found that the LSPR peak red shifted linearly with the refractive index of the solvent when using solvents that do not possess active functional groups that could complex with the surface of the Au NPs. A nonlinear relationship between the LSPR peak position and refractive index was found, however, when using solvents with nonbonding electrons capable of complexing to the surface of the Au NPs. Interestingly, these authors found that the LSPR peak position blue shifted ~ 3 nm for every one carbon atom when the NPs were dispersed in alcohols with varying linear carbon chain lengths.26 This rather atypical trend reverses in the presence of more strongly coordinating ligands. In such cases, the magnitude of the red shift caused by stabilizing ligands, such as alkyl amines or thiols, increases when the headgroup of the ligand interacts more strongly with the surface of the Au NPs. In a recent study,28 a dispersion of gold nanorods (GNRs) with different aspect ratios was used to perform a multiplexed bioanalytical sensing measurement in solution. This work exploited the fact that small changes in the aspect ratio of GNRs lead to drastic changes in their optical properties (the longitudinal plasmon mode red shifts with increasing aspect ratio, as described in section 2). A series of GNRs with aspect ratios (length/width) of 2.1, 4.5, and 6.5-annotated as GNR 1, 2, and 3, respectively-were functionalized with a different recognition molecule (Figure 15a) and dispersed into a single solution. 286 The solution vielded a composite absorption spectrum with three pronounced longitudinal LSPR maxima, each one corresponding to one of the three types of GNR (Figure 15b). The longitudinal peaks, in order

已完成理解「请为以下文章生成一篇简短的英文演讲稿，内容不超过500词。4.4. Label-Free Optical Detection Based onChanges in Refractive Index Plasmons are exceptionall sensitive reporters for chemical phenomena that influence the refractive index of the local environment of a probe. SPR spectroscopy and imaging are well-known label-free optical detection methods that use this property to monitor surface binding events in real time (as discussed in a separate contribution in this issue). In these techniques, changes in the dielectric environment at the surface of a flat or periodically structured noble-metal film shift the observed SPR resonance, which can be measured using angular interrogation, wavelength interrogation, or intensity measure-ments. This well-established technique enables determination of kinetic and thermodynamic data for a wide variety of molecular binding events, especially those involving bio-molecular targets.371,374,375 Progress is currently being made to develop nanostructured plasmonic materials for performing similar analyses. Changes in the dielectric environment of nanostructured metals, such as NPs, results in measurable shifts of the LSPR peak position and/or magnitude that can be used to perform label-free chemical or biosensing in real time. A varietv of noble-metal nanostructures such as NPs in solution or immobilized on surfaces, nanoholes and nanohole arrays, and nanoisland films379-381 have been used in this way. Substrate-bound nanostructures offer several attractive features as platforms for chemical sensing. These include the shape, size, composition, and spacing of the NPs can be readily controlled to provide tunable peak positions and widths and (ii) the NPs are free of the capping agents or stabilizers used in solution-phase NP synthesis, making their surfaces readily accessible for functionalization with specific receptors or ligands. In the following discussion we will provide some examples of chemical and biosensing based on refractive index changes near the surface of nanostructured metals in solution and on substrates, where the LSPR is monitored using extinction or scattering spectroscopy. The refractive index sensitivity of each sensing platform will also be given where validated data is available. This sensitivity is commonly defined in terms of the change in an experimentally measurable parameter (typically peak position or magnitude per 'refractive index unit' (RIU), which corresponds to a change of 1 in the refractive index. These measurements are usually performed by taking spectra of a plasmonic nanostructure in solutions of increasing refractive index while monitoring peak position or intensity changes. 4.4.1. Nanoparticle Dispersions Au nanostructures are known to exhibit strong plasmonic bands that are dependent on their shape, size, and surrounding media. Ghosh et al.26 studied the effects of changing solvents and ligands on the LSPR of Au NPs dispersed in solution. It was found that the surface plasmon absorption maximum of the Au NPs varied between 520 and 550 nm, depending on the refractive index and chemical nature of the surrounding solvent. The authors found that the LSPR peak red shifted linearly with the refractive index of the solvent when using solvents that do not possess active functional groups that could complex with the surface of the Au NPs. A nonlinear relationship between the LSPR peak position and refractive index was found, however, when using solvents with nonbonding electrons capable of complexing to the surface of the Au NPs. Interestingly, these authors found that the LSPR peak position blue shifted ~ 3 nm for every one carbon atom when the NPs were dispersed in alcohols with varying linear carbon chain lengths.26 This rather atypical trend reverses in the presence of more strongly coordinating ligands. In such cases, the magnitude of the red shift caused by stabilizing ligands, such as alkyl amines or thiols, increases when the headgroup of the ligand interacts more strongly with the surface of the Au NPs. In a recent study,28 a dispersion of gold nanorods (GNRs) with different aspect ratios was used to perform a multiplexed bioanalytical sensing measurement in solution. This work exploited the fact that small changes in the aspect ratio of GNRs lead to drastic changes in their optical properties (the longitudinal plasmon mode red shifts with increasing aspect ratio, as described in section 2). A series of GNRs with aspect ratios (length/width) of 2.1, 4.5, and 6.5-annotated as GNR 1, 2, and 3, respectively-were functionalized with a different recognition molecule (Figure 15a) and dispersed into a single solution. 286 The solution vielded a composite absorption spectrum with three pronounced longitudinal LSPR maxima, each one corresponding to one of the three types of GNR (Figure 15b). The longitudinal peaks, in order 」