## 6 Conclusions  
Fresh products emit VOCs in response to compositional changes, environmental conditions, and microbial contamination. Fungi and bacteria are the main causal agents of the contamination and the deterioration of agricultural products, with fungi being the most predominant. These use different modes of action to establish themselves on their host. Some authors have reported one or several volatile biomarkers for detecting microorganisms, while others prefer to associate them with a functional group based on its \begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Volatile** Biomarker & **Biosensor** & **Result** & **Reference** \\ \hline Carbon dioxide. & The biosensor prototype is based on CuO nanoparticles and nanolayers. & The carbon dioxide released by _Aspergillus niger_ was detected by a biosensor prototype based on layers of CuO nanoparticles deposited on a glass substrate with silver adhesive, onto which the fungus was placed, measuring the resonance with an amperemeter. As the growth rate of the fungus increased, resistance decreased. The biosensor contained a carbon nanotube electrode modified with tyrosine, whose interaction with p-ethylphenol resulted in cyclic voltammetry and constant potential ampereometry. The biosensor showed a high sensitivity of 4.0 \(\pm\) 0.5 \(\upmu\)A cm\({}^{-2}\) \(\upmu\)M\({}^{-1}\) by constant strawberry fruit. & [66] \\ \hline \multirow{6}{*}{p-ethylphenol} & Enzymatic biosensor to detect _Phtyophthora_ & The biosensor contained a carbon nanotube electrode modified with tyrosine, whose interaction with p-ethylphenol resulted in cyclic voltammetry and constant potential ampereometry. The biosensor showed a high sensitivity of 4.0 \(\pm\) 0.5 \(\upmu\)A cm\({}^{-2}\) \(\upmu\)M\({}^{-1}\) by constant strawberry fruit. & [67] \\  & & Potential ampereometry with a detection limit of 0.10 \(\pm\) 0.02 \(\upmu\)M and a quantification limit of 0.29 \(\pm\) 0.07 \(\upmu\)M. & [68] \\  & & The biosensor used alcohol oxidase and horseradish & \\  & & peroxidase as receptors. They were placed on a glassy carbon electrode-modified surface with multi-walled carbon nanotubes. The interaction resulted in an amperemetric signal proportional to methyl salicate released by _Tetramiculus vitate_, _Fusarium_, and _Phtyophthora capsici_ during infection. The sensitivity of the biosensor was 112.37 and 282.82 \(\upmu\)A cm\({}^{-2}\) mM\({}^{-1}\) by cyclic ampereometry and constant potential ampereometry with a detection limit of 2.95 \(\upmu\)M and 0.98 \(\upmu\)M, respectively. & [69] \\  & & Esterase, salicate hydroxylase, and tyrosinase were used as biomarkers for the detection of methyl salicate in a biosensor based on the reduction in 1,2-benzquinone to catechol by the action of the enzymes. The resulting reaction was measured ampereometrically. The biosensor exhibited a sensitivity of 1.5 \(\upmu\)A cm\({}^{-2}\) and with a detection limit of 0.7 \(\upmu\)M. & [68] \\ Ethylekanol, linalool, tetradace, and phenylacetaldehyde with single-stranded DNA & Biosensor based on single-walled carbon nanotubes functionalized with single-stranded DNA & [69] \\  & & The biosensor managed to detect the compounds in a saturation range between 5 and 100\%, having a resistance error in terms of its reproducibility of less than 10\%. & [69] \\  & & Paciillium digitatum growth on ranges was detected using an optical biosensor that captured the luminescence emitted by the bacteria due to the changes in VOCs and concentration after infection. The biosensor was able to detect the luminescence of the bacteria on the third day of infection before the first visible symptoms appeared on the fruit. & [70] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Biosensors to detect phytopathogens through VOCs. variability, depending on the substrate and the host. In all cases, the concentration of these VOCs is very low (in the order of ppb), so it is necessary to use precise analytical techniques such as GC-MS and PTR-MS.