Light analyte interaction

Freely suspended liquid droplets are characterized by their shape determined by surface tension leading to ideally spherical shape and smooth surface at the subnanometer scale. These properties suggest liquid droplets as optical resonators with extremely high quality factors, limited by material absorption. Liquid microdroplets have found a wide range of applications for cavity-enhanced spectroscopy and in analytical chemistry, where small volumes and a container-free environment is required for example for protein crystallization investigations. This chapter reviews the basic physics and technical implementations of light-matter interactions in liquid-droplet optical cavities. 

The matrix in MALDI MS fulfils several essential functions. First, the matrix absorbs the laser light via electronic (UV-MALDI) or vibrational (IR-MALDl) excitation and transfers this energy smoothly onto the analyte. Due to the high molar excess of the matrix over the analyte, the intermolecular interactions of analyte molecules are reduced, thus facilitating transfer into the gas phase. Last but not least, matrix-analyte interactions play an active role both in the ionization of the analyte as well as in its desorption [34]. 

It is common practice to classify the optical sensors according to the type of effect produced by the receptor-analyte interaction in the incoming light beam. 

There are two ways in which the intensity of light in the optical fiber can be modulated by the analyte using absorption, (a) direct absorption by the analyte and (b) indirect absorption, where another material acts as a transducer, such as electroactive polymer or sensitive dye. The analyte interacts with this material layer changing its absorbance, and hence the intensity of light in the optical fiber. 

Indirect absorption is based on using another material (or medium) as the sensor transducer. The analyte interacts with this material changing its absorbance, and hence, the intensity of light interacting with this material. 

All previously described analytical tools and specifications are used to describe the quality of a biopharmaceutical. In terms of use, it is important to know how stable the product itself and its formulation are, and which specific handling conditions or precautions must be estabhshed. Stabihty studies are conducted to generate the corresponding data [26]. These studies address (apart from temperature and humidity) light exposure, interaction with the container/closure system (e.g., vials or ready-to-use syringes) and also in-use stability, for examples in case of dissolved lyo-philisates. 

Although the thermodynamic terms derived from Eq. (1) can also be used to shed light on recognition of an analyte by an MIP solvent-analyte and analyte-analyte interactions and macroscopic effects (e.g., solvent and analyte diffusion rates and surface areas) also play a critical role in determining the binding characteristics of a polymer and its suitability for use in a given application. 

Figure 5.5 LSPR detection schemes, (a) nanoparticle assay and (b) nanoparticle substrate. Usually, the incident light is a white light the optical absorbance spectra through the particles before and after analyte interaction are measured.

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