1.6 Detection Methods

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Figure 1.49: Gel permeation chromatography (GPC), also called size-exclusion chromatography (SEC). The

solid phase of the column contains pores of a specific size. Molecules of a size smaller than the pores are

often retained, and thus are eluted last.

synthesized in the presence of a small molecule template. The template is then washed

out, but the imprint specific to the template stays behind. This method was originally de-

signed to separate out enantiomers but now has also been developed for enzyme mimics

and other applications. In all chromatography, the different fractions are collected and

then analyzed by a variety of methods and detectors.

There are a large variety of detectors. Most of them are based on weight, light, pres-

sure, or electricity output. For nanotechnology applications, the most common detector

based on weight is the quartz crystal microbalance (QCM) (Figure 1.50). It does not really

measure the weight of the crystal but its resonance frequency, and the resonance fre-

quency is dependent on the weight of the sample. So, if something binds to the surface

of the crystal, a shift in resonance frequency is detected.

There are a variety of detectors based on light (Figure 1.51a). Light absorbance can

be measured with an UV/Vis spectrometer, light emission by a fluorimeter. Absorbance is

based on the composition and concentration of a sample. Therefore, the absorbance can

also be used to measure concentration using Beer’s law. Fluorescent molecules, when

excited with a specific wavelength, emit light in a different and higher wavelength (Fig-

ure 1.51b). Fluorescence is used in a variety of ways. There are fluorescent dyes that

change their emission spectrum when they are in different phases. That can be used

to measure the formation of a specific lipid phase. Fluorescence is often used to detect

the location and concentration of a metabolite in a cell by using fluorescent antibodies

(Figure 1.51c). Luminescence can be used similar to fluorescence. Specific fluorescent

or luminescent dyes are used; dyes for a large variety of experiments have been devel-

oped.