SENSORS BASED ON MONOLAYER-CAPPED METALLIC NANOPARTICLES 153 exist simultaneously in a given system. Usually, however, one sensing mechanism dominates and deter- mines the sensor's performance. 4.3. THE ROLE OF THE NUMBER OF NANOPARTICLES IN CHEMICAL SENSING It is still controversial whether using a large number of MCNPs is of advantage in chemical sensing. Systematic studies of the correlation between the number of elements (i.e., MCNPs) and the sensing performance are necessary to solve this query. It might also be possible to clearly identify the type and characteristics of interaction between the analyte(s) and MCNPs, if the number of MCNPs could be obtained with suffi cient precision. However, in practice, it is rather challenging to determine the exact number of MCNPs, and to date, it is not yet feasible to obtain such highly precise numbers of MCNPs in realistic sensing experiments. 5. CATEGORIES OF MCNP-BASED CHEMICAL SENSORS Implementations of MCNP-based sensors can be quite diverse. MCNP elements that will be discussed in this section are based on optical, electrical, electrochemical, and piezoelectric transduction approaches. While the focus will be on the sensing mechanism of the various MCNP sensors, we will also provide a number of examples for their practical application. These include chemical and biochemical processing, quality control in the food industry, environmental monitoring, transportation, etc. In each of the following sections, a brief description of the working principle of a given class of MCNP-based sensors, supported by pertinent examples of actual developments, will be provided. Insight gained by these studies into the fundamental and sensing properties of MCNPs will be discussed. 5.1. OPTICAL SENSORS Optical sensors respond with a measurable change of their optical properties to one or more external stimuli, such as temperature (Hotate 2006), stress (Hotate 2006), biocoating, or changes in the chemical environment. Hence, an optical sensor can be tuned, by suitable design, to identify a particular analyte or to distinguish between groups of analytes, using one or more signal transduction mechanisms, for example, changes in absorbance and reflectance spectra (Wiki and Kunz 2000; Caglar et al. 2006), in fluorescence intensity and lifetime (Wolfbeis 2005), wavelength shifts (Wiki and Kunz 2000; Riza et al. 2007), changes in spectral shape (Bussetti et al. 2005), or in the polarization state of reflected or trans- mitted light (Shirshov et al. 1998). 5.1.1. Surface Plasmons in Metal Nanoparticles Metal NPs have particularly interesting optical properties, which stem from their unique interaction with light, and are manifest in their intense colors, as indicated below in Figures 4.3 and 4.4 (Jain et