# Fluctuation-Enhanced Sensing (FES): A Promising Sensing Technique
## 6 Reducing Errors and Enhancing the Sensory Information Channel Capacity  
### Photonic Excitation  
As we have mentioned before, conductance noise spectra are the superposition of over-damped Lorentzian spectra, which means no sharp peaks in the FES spectrum can be present. However, photonic excitations combined with FES measurements have the potential to increase the information content of measurements. With properly selected photon energies (wavelengths) some of the otherwise dormant states can be activated and they can produce a modified spectrum with new information. UV Light Excitation Experiments and ResultsA series of low-cost UV LEDs, emitting UV light within wavelet range 250-355 nm is available on the market. Resistive gas sensing layer can be made on ceramic or silicon substrates and efficiently irradiated from different distances. Selected wavelengths of the UV light can modulate the physical properties of the gas sensing layer (e.g., DC resistance, low-frequency resistance fluctuations) [85]. It has been observed that changes of DC resistance induced by UV irradiation are independent of the changes of the recorded low-frequency noise [86]. Thus, both phenomena secure additional information about the ambient atmosphere and can be utilized to improve gas detection. During FES experiments 1/_f_ dependence with a plateau at selected corner frequencies, characteristic for an ambient gas and the wavelength of the applied UV LEDs, was observed. The intensity of the 1/_f_ noise component was also informative and depended on gas concentration. The FES method was advantageous at low gas concentrations enhancing threshold gas detection. This effect was observed at an ambient atmosphere of different gases. It is a valuable result for practical applications, especially for medical diagnosis by exhaled breath analysis when deficient gas concentrations have to be detected. Moreover, faster response of the gas sensor was observed. This effect is rather evident because UV light was formerly applied to cleanse the sensor. UV irradiation replaced a heating pulse, commonly employed for fast sensor cleansing. We should underline that a photocatalytic effect in gas sensing depends on a few independent factors:  
* morphology of gas sensing layer, determining the depth of light penetration,
* irradiation intensity,
* the wavelengths of the UV light. As we see (Figure 5), the gas sensing layer is a porous grainy material. The gas molecules can penetrate the pores of the whole gas sensing layer due to the diffusion process and change DC resistance or low-frequency resistance noise recorded between the terminals. The UV light penetrates only the thin part of the gas sensing layer. The depth of penetration depends on light wavelength, irradiated material and its morphology. We can assume that the depth of UV light penetration does not exceed a few \(\upmu\)m [85]. Thus, the gas sensing layer can be modelled as two parallel resistors: an external one modulated by UV light and the inner one without this effect. It was observed that there is a threshold intensity of UV light when its further intensification does not induce the changes in the gas sensing layer. This effect is a result of a limited impact on the thin sensing layer only. The low-cost gas sensor in Figure 6, made of a mixture of graphene flakes (Graphene Supermarket UHC-NPD-100 ML) and TiO\({}_{2}\) nanoparticles (AEROXIDE(r) TiO\({}_{2}\) P25) by painting and baking, has a thickness of about 100 \(\upmu\)m, which is about 100 times more than the estimated depth of UV light penetration. Thus, UV light affects less than 1% of its porous and gas-sensitive volume. Figure 6: Gas sensing layer comprising TiO\({}_{2}\) nanoparticles and graphene flakes: photo of the sensor made by painting on a silicon substrate with four-point gold contacts. The easiest way of increasing UV light modulation is to apply a thinner gas sensing layer. Moreover, a thinner layer will secure a faster gas response, which is very important for any practical application.