This Review covers photonic crystals (PhCs) and their use for sensing mainly chemical and biochemical parameters, with a particular focus on the materials applied. Specific sections are devoted to a) a lead-in into natural and synthetic photonic nanoarchitectures, b) the various kinds of structures of PhCs, c) reflection and diffraction in PhCs, d) aspects of sensing based on mechanical, thermal, optical, electrical, magnetic, and purely chemical stimuli, e) aspects of biosensing based on biomolecules incorporated into PhCs, and f) current trends and limitations of such sensors.

Graphene as a transducer material has produced some of the best-performing sensing approaches to date opening the door toward integrated miniaturized all-carbon point-of-care devices. Addressing this opportunity, laser-scribed graphene (LSG) electrodes are demonstrated here as highly sensitive and reliable biosensor transducers in blood serum analysis. These flexible electrodes with large electrochemical surface areas were fabricated using a direct-write laser process on polyimide foils. A universal immobilization approach is established by anchoring 1-pyrenebutyric acid to the graphene and subsequently covalently attaching an aptamer against the coagulation factor thrombin as an exemplary bioreceptor to the carboxyl groups. The resulting biosensor displays extremely low detection limits of 1 pM in buffer and 5 pM in the complex matrix of serum.

Monodisperse, highly negatively charged, cross-linked polystyrene nanoparticles with diameters between 80 and 120 nm have been incorporated into a polyacrylamide hydrogel, where they display an iridescent color that conventionally is attributed to the so-called photonic crystal effect. The film is of red color if placed in plain water but turns to green in the presence of a 1 mM solution of an electrolyte such as sodium chloride and to purple in 100 mM solutions of electrolytes. Quantitative reflection spectroscopy was performed at various wavelengths and resulted in plots of reflected light wavelength versus ionic strength (IS) that are almost linear in the logarithmic concentration range from 5 × 10(-5) to 10(-2) mol·L(-1). We show that such films are capable of monitoring the IS of aqueous solutions in the pH range from 5 to 9. We also show that, in addition to visual and instrumental readout, the sensor films can be analyzed with a digital camera at fixed angle. The digital images were separated into their red, green, and blue channels and analyzed. The red channel was found to be best suited for determination of the IS and resulted in calibration plots that are comparable if not better than those obtained by reflectometry.

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Monodisperse polystyrene nanoparticles with a diameter of 173 nm were incorporated into a polydimethylsiloxane matrix where they display an iridescent color that can be attributed to the photonic crystal effect. The film is of violet color if placed in plain water, but turns to red in the presence of the non-polar solvent n-hexane. Several solvents were studied in some detail. We show that such films are capable of monitoring the water content of ethanol/water mixtures, where only 1% (v/v) of water leads to a shift of the peak wavelength of reflected light by 5 nm. The method also can be applied to determine, both visually and instrumentally, the fraction of methanol in ethanol/methanol mixtures. Here, a fraction of 1% of methanol (v/v) results in a wavelength shift of 2 nm. The reflected wavelength is not influenced by temperature changes nor impeded by photobleaching. The signal changes are fully reversible and response times are <1 s.