Spectroscopy, noninvasive surface methods

Noninvasive surface spectroscopies can be applied in the presence of liquid water most of them involve the input and detection of photons. The best known examples are nuclear magnetic resonance, electron spin resonance, Raman, Fourier transform infrared, UV-visible fluorescence, X-ray absorption, and Mossbauer spectroscopies, although Brown (28) enumerated many others that are available to detect adsorbed ions. These methods, some of which are listed in Table II along with citations of illustrative applications, can be used both noninvasively and in conjunction with in situ probes. 

In recent studies on perfused rats hearts (Veitch et al., 1992), it was found that differences in the sensitivity of complexes 1-lV to ischaemic damage were dependent upon the duration of ischaemia and the presence of oxygen. The demonstration that complex 1 is a major defective site dependent upon isolation of mitochondria from homogenates of the tissue by in vitro methods seemed important to us. We therefore decided to attempt to make noninvasive measurements of mitochondrial function soon after reperfusion in transplanted rabbit kidneys by surface fluorescence (for mitochondrial NADH levels) and near infra-red spectroscopy (NIRS) for the redox state of cytaas. 

The molecular spectroscopy of adsorbed cations and anions has two principal subdivisions (1) invasive methods (such as X-ray photoelectron or secondary-ion mass spectrometry) that require sample desiccation and high vacuum and (2) noninvasive methods that require little or no alteration of a sample from its received condition. Invasive methods have an important role to play in the characterization of solid surfaces (27), but to use them for resolving surface speciation on particles in aqueous systems simply begs the question. 

These illustrative examples of ESR and EXAFS spectroscopy applied to detect surface complexes demonstrate the typical use of noninvasive methods but also expose what remains problematic about the information provided by these methods. Stumm et al. (4) wisely retained the Stem dichotomy for strongly adsorbed metals and considered surface complexes that contain both solvated and desolvated metal ions. This dichotomy persists in spectroscopic data of metal adsorption on hydroxylated surfaces. Often, they can give clear evidence for immobilization of a metal ion on a surface within the time scale of the spectroscopic method, but not necessarily for complete desolvation of the ion to form an inner-sphere complex with surface hydroxyl groups. 

In many pharmaceuhcal and biomedical applicahons, such as the noninvasive probing of colored capsules, coated tablets and subsurface hssue spectroscopy, it is also beneficial that the transmission Raman method very elfechvely diminishes the Raman and fluorescence signals originating from the surface layers of the probed sample compared to conventional backscattering Raman geometry [37]. 

Abstract Optical detection continues to dominate detection methods in microfluidics due to its noninvasive nature, easy coupling, rapid response, and high sensitivity. In this review, we summarize two aspects of recent developments in optical detection methods on microfluidic chips. The first aspect is free-space (off-chip) detection on the microchip, in which the conventional absorption, fluorescence, chemiluminescence, surface plasmon resonance, and surface enhanced Raman spectroscopies are involved. The second aspect is the optofluidic (inside-chip) detection. Various miniaturized optical components integrated on the microfluidic chip, such as waveguide, microlens, laser, and detectors are outlined. 

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