## 4 Conclusions  
In this paper, we have successfully demonstrated the fabrication and operation of a novel amperometric screen printed biosensor for the measurement of linoleic and \(\alpha\)-linoleic acid down to 24 and 100 nM, respectively. This is mainly due to electrocatalytic responses of hydroperoxides of the PUFAs, produced by LOX; the oxidation potentials of these species occur at much lower values than obtained by direct oxidation at plain SPCEs. The fabrication method is convenient and inexpensive; it requires only deposition of an enzyme (LOX) and glutaraldehyde layer onto the surface of a CoPC-SPCE. These single use, disposable biosensors are used in conjunction with amperometry in stirred solution, which is simple to perform and easy to interpret, thus suitable for use by non-technical personnel. The response is proportional to concentration across a wide range, and linear from 0.2 to 10 \(\upmu\)M, thus has potential for measurement FFAs for food safety and quality purposes. It should be mentioned that, in addition to CoPC, other cobalt species have been successfully employed as the basis of electrocatalytic amperometric (bio)sensors. For example, [PERSON] et al. [22] successfully developed an amperometric biosensor based on cobalt hexacyanoferate for glucose determination using modified carbon film electrodes with glucose oxidase; [PERSON] et al. [23] developed an enzyme-less amperometric sensor based on cobalt oxide nanoparticles for the measurement of glucose, achieving detection limits down to 5 nM and cobalt chloride was immobilised on a glassy carbon electrode using chitosan and MWCNTs by [PERSON] et al. [24] for paracetamol determination. However, to our knowledge, none of these cobalt species have been investigated or employed in sensors for fatty acid analysis. The performance of our biosensor compares favourably to other electrochemical approaches for measuring unsaturated FFAs (Table 3). [PERSON] et al. (2018) [3] used a cobalt phthalocyanine (CoPC) mediated carbon paste electrode to measure linoleic acid in safflower oil. The linear range was 7.5 to 200 \(\upmu\)g/mL and the detection limit was 2.5 \(\upmu\)g/mL. [PERSON] et al. developed an amperometric biosensor containing LOX, for the measurement of linoleic and \(\alpha\)-linoleic acid in fats and oils, using an  
Figure 6: Amperometric responses at a cobalt phthalocyanine screen-printed carbon electrode with 15 units of lipoxygenase, obtained following 5 additions of 0.2 \(\upmu\)M linoleic acid, with standard addition plot (insert). oxygen electrode as the base transducer [25]. The detection limits were 7.7 and 4.8 uM, respectively, with a linear range of 0.01 to 0.2 mM for both fatty acids. [PERSON]'s method did not include an enzyme, and was only able to measure a single PUFA. Carbon paste and oxygen electrodes also problematic for commercialisation. For commercialisation purposes biosensors need to be mass produced at low cost. [PERSON] et al. [26] used a screen printed graphene oxide biosensor containing LOX and ruthenium electrocatalyst to measure the monounsaturated fatty acid oleic acid in calf serum. The detection limit was \(<\)0.1 mM, and the linear range was 0.1 to 1.0 mM. The authors claim it is very selective for oleic acid, which has a different reaction mechanism with LOX than PUFAs, which were not measured. We believe that our approach offers a real possibility of achieving a commercial device for the measurement of PUFAs; future work will investigate the application this novel biosensors to food samples. Conceptualization, A.S., A.C., O.D. and J.P.H. ; formal analysis, A.S., A.C., O.D. and J.P.H. ; funding acquisition, A.S., A.C., O.D. and J.P.H. ; investigation, A.S., A.C., O.D. and J.P.H. ; methodology, A.S., A.C., O.D. and J.P.H. ; project administration, A.S., A.C., O.D. and J.P.H. ; supervision, A.S., A.C., O.D. and J.P.H. ; validation, A.S., A.C., O.D. and J.P.H. ; visualization, A.S., A.C., O.D. and J.P.H. ; writing--original draft preparation, A.S., A.C., O.D. and J.P.H.