Encyclopedia of Microfluidics and Nanofluidics

Living Edition
| Editors: Dongqing Li

In Situ Infrared Spectroscopy

  • Karsten HinrichsEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27758-0_1776-4

Definition

In situ infrared (IR) spectroscopy is a spectroscopic method for the infrared spectral range which can be used in defined environments during preparation, modification, function, and reaction or analysis in natural environment. In this contribution especially liquid environments are considered with the focus on the mid-infrared (MIR) spectral range from 2.5 to 16 μm.

Overview

In situ IR spectroscopy is used for various studies from technologically relevant films [1, 2, 3, 4, 5, 6] towards cells [7] in liquid environments. The most common in situ IR techniques for characterizations of thin films and surfaces at the solid-liquid interface are attenuated total reflection (ATR), polarization-dependent reflection absorption IR spectroscopy (RAIRS), and infrared spectroscopic ellipsometry (IRSE) [1, 2, 3, 4, 5, 6, 8]. These techniques are applied in current research on solid-liquid interfaces of functional films and surfaces as well as cells. Examples are titration, electrochemical etching of silicon oxide, electrochemical growth of organic films, as well as the stimuli-responsive switching and protein adsorption behavior of thermoresponsive or pH-responsive polymer brushes. As an example, Fig. 1 shows (according to [3]) spectra of two pH-dependent reversible switching states of an 11 nm thick binary polymer brush. The high potential of IR spectroscopy/ellipsometry relies on the fact that thickness, chemical and structural changes, and adsorbed or desorbed amount of molecules but also the water content in films can be analyzed via the interpretation of absorption bands of specific molecular vibrations by the use of adequate optical models. In situ IR microscopy might be used for measurements with lateral resolutions in the μm range allowing, e.g., the chemical imaging of live cancer cells in aqueous environments [6]. Using synchrotron radiation sources, IR microscopy was also used for studies with microfluidic devices [7].
Fig. 1

Infrared ellipsometric spectra for pH = 2 and pH = 10 (Data were taken from Mikhaylova et al. [3]). The binary PAA-PVP brush has a thickness of 11 nm. A sketch shows schematically the different swelling states of the brush

Methodology

Introduction

In situ spectroscopy in the mid-infrared spectral (MIR) range can be performed through a thin layer of solution; however, depending on the penetration depths of radiation in the specific liquid, the thicknesses of these liquid films are limited. Then typical liquid film thicknesses lie in the micrometer range, which would make it difficult to build up flow cells. Therefore, it is more typical to study the film of interest from the backside through an IR transparent substrate (e.g., ZnSe, silicon) which is in contact with the liquid reservoir of the measurement cell. For the latter case, several geometries exist in single or multiple reflection geometries (see Fig. 2). Depending on the geometry of the cell and prism, incidence angles below or in the attenuated total reflection (ATR) regime are used. Often, the ATR geometry limits the accessible spectral range due to the longer path of radiation through the prism (see, e.g., Andanson and Baiker [9]). Figure 2 compares different possible geometries for in situ IR spectroscopy. However, besides these general geometries, also more complex microstructured surfaces might be used. Figure 2a shows a single reflection geometry below the ATR regime in which the path through the substrate can be minimized, for silicon, e.g., spectral ranges down to 900 cm−1 can be analyzed. The methodology can be combined with standard FTIR but also dispersive IR spectroscopy. As light sources, all available light sources from globar to lasers could be used. Brilliant light sources have the advantage that the incidence angle is better defined at high optical throughput and smaller measurement cells or higher lateral resolutions can be used. Setups can also be combined with SEIRA measurements [10].
Fig. 2

Possible geometries for in situ cells: (a) single reflection below ATR regime; (b) single reflection in ATR regime; (c) multiple reflections in ATR geometry. The optical path (red) through the transparent material (grey) is shown. The probed film (dark blue) is in contact with the liquid reservoir (light blue)

Key Research Findings

Cell geometries as given in Fig. 2 are successfully applied in current research for chemical, physical, and biotechnological applications. A single reflection ATR geometry (Fig. 2b), for example, was implemented in an electrochemical cell and also used for the study of dissociation of carboxylic groups [2]. Also for studies of liver cells in combination with an IR microscope, a single reflection ATR crystal was used. Single reflection measurements with polarization-dependent spectroscopy (Fig. 2a) and ellipsometry were performed in a reflection cell below the ATR regime for the monitoring of electrochemical etching and growth of polymeric films, modification of pH- and temperature-responsive polymer brushes as well as protein adsorption thereon [3, 5]. Multiple reflection geometries (Fig. 2c), e.g., were established for catalytic studies [9] and voltammetric studies [10].

Future Directions

In situ IR spectroscopy is interesting for a broad range of technological applications, e.g., in (i) biomedicine and biochemistry, (ii) electrochemistry, (iii) catalyses, and (iv) microfluidic devices. In particular, it is relevant for the design of functional templates for drug release [5], studies of smart films and surfaces [11], characterization of living cells in contact with solution [6], control during electrochemical preparations [4], monitoring and understanding of catalytic processes [9], and microfluidics. It can be expected that advanced infrared technology, e.g., new IR lasers and detectors, bring advances in the used infrared methods, e.g., by the use of the new technology in combination with smart sensing devices or microfluidic devices.

Cross-References

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Copyright information

© Springer Science+Business Media New York 2013

Authors and Affiliations

  1. 1.Leibniz-Institut für Analytische Wissenschaften - ISAS - e.V.BerlinGermany