## 5 Sensing Methods for Formaldehyde
### Fish  
Really different from the meat matrix is the fish matrix in terms of papers present in literature: Table 3 shows the analytical performances of papers published in recent years (from 2015). The reason is possibly due to the fact that for a long time FA has been used as a food additive (E240) for the preservation of crustaceans (in which can be present up to values of 100 ppm) and smoked products (with higher values, up to 1000 ppm). Further, the other important reason is due to the fish degradation: after 7-10 days, or earlier if the temperature is above 0 \({}^{\circ}\)C, the first important alterations of the fish begin. Initially, it could be a witness to the transformation of triethylamine oxide into trimethylamine and, subsequently, into dimethylamine by bacterial and endogenous enzymes. Over time, the reaction continues, leading to the formation of monoethylamine and formaldehyde (responsible for the typical smell of spoiled fish). For these two reasons, fish is a well-studied matrix by means both of sensing methods [48; 49; 54; 55; 56; 57; 58; 59; 60; 61] and no-sensing methods [62; 63; 64; 51], especially recently favoring in situ approaches. In this contest squid is one of the main matrices investigated: over [PERSON] et al. [48], whose methodology was just described above, other authors have dealt with this determination in the last few years. [PERSON] et al. [54] reported the development of an electronic nose (e-nose). The e-nose system was made by three groups of sensors, for a total of 18 sensors, showing different responses to volatile compounds in squid. The coating material of the P-type sensor was SnO\({}_{2}\), whereas for T type sensor the coating materials were Pd and Pt, and for the LY-type sensor, they were Cr\({}_{2}\)O\({}_{3}\) and Ti. The authors stated to achieve rapid (120 s) and quantitative (no analytical parameters are reported in the paper except RSDs, <0.143) FA determination whereas GC-MS analysis was used by the authors for validating the method. [PERSON] et al. developed an electrochemical sensor made of the homogeneous distribution of palladium nanoparticle (PdNPs) on poly (acrylic acid)-functionalized graphene oxide (PAA-GO) modified on a glassy carbon electrode (GCE) (PdNPs-PAA-GO/GCE) with incorporated flow-injection amperometry (FI-Amp) [55]. TEM and Fourier transform infrared spectroscopy (FTIR) were used for characterizing the surface sensor morphology whereas cyclic voltammetry was involved in the measurements (working potential from \(-\)0.6 V to +0.7 V with scan rate 0.05 V s\({}^{-1}\)). The samples (i.e., squid, apple, Chinese cabbage, and cabbage) were just subjected to treatment with acetyl acetone reagent for 2 h. The authors discussed the electrochemical characterization: the formalin oxidation was obtained at \(-\)0.07 V where the sensor showed good electrocatalytic activity. Further, they addressed information about the effect both of PdNPs-PAA-GO amount (40 \(\upmu\)g), the applied potential (\(-\)0.25 V), the flow rate (0.500 mL min\({}^{-1}\)), and the sample volume (250 \(\upmu\)L) using these parameters, the authors achieved really interesting goals: an r\({}^{2}\) of 0.9997 in the range 50-50,000 mmol L\({}^{-1}\), LOD and LOQ of 16 mmol L\({}^{-1}\) and 53 mmol L\({}^{-1}\), respectively, and recoveries ranging between 94 and 104.2% with an RSD < 3.5% and no effects on the measures due to interfering ions (i.e., Cl\({}^{-}\), K\({}^{+}\), Na\({}^{+}\), NH\({}_{4}\)\({}^{+}\), NO\({}_{3}\)\({}^{-}\), CO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), PO\({}_{4}\)\({}^{3-}\)). [PERSON] and [PERSON], on the other hand, developed a room-temperature gas sensor based on 2D hybrid pristine, NH\({}_{2}\) and N\({}_{2}\) functionalized multi-wall carbon nanotubes (MWCNTs)/PEDOT:PSS conductive polymer [56]. The peculiarity of this sensor is that the authors made it by inkjet printing technique: they managed to control the thickness of printed sensing films by overwriting of sensing ink on the substrate.