# A Photonic Label-Free Biosensor to Detect _Salmonella_ spp. in Fresh Vegetables and Meat
## 3 Results and Discussion
### Sensor Selectivity and Efficiency  
The detection efficiency of the immunosensor method for _Salmonella_ spp. was assessed at concentrations ranging from 10 to \(10^{10}\) CFU/mL. To determine the most effective enrichment method for _Salmonella_ spp. detection, a variety of _Salmonella enterica Enteritidis_ and _Salmonella enterica Typhimurium_ strains were used, with samples of red Batavia and fresh chicken spiked with these strains. Naturally contaminated samples (fresh salad and fresh chicken) were also used for evaluation. Validation involved analyzing the presence or absence of the pathogen and comparing the detection units of the immunosensor with the results of plate culture methods. Results are summarized in Tables 1 and 2. The evaluation was performed using the equal detection method on identical samples employing various measurement equipment by different operators. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Target Species \({}^{*}\)** & **Immunosensor Response \({}^{**}\)** &
\begin{tabular}{c} **Count on Selective Agar** \\ (CFU/mL) *** \\ \end{tabular} \\ \hline _Salmonella enterica_ & & \\ _Typhimurium_ CECT4266 & + & \(1.5\times 10^{10}\) \\ 04/2023 & & \\ _Salmonella enterica_ & & \\ _Typhimurium_ CECT4266 & \(-\) & 0 \\ 04/2023 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of results for target species. Figure 7: **Comparison of results of immobilization between activated and non-activated surfaces using EDC/NHS. ** These results reflect fluorescence intensity for each surface, where both polyclonal (pAb) and monoclonal (mAb) antibodies against _Salmonella_ spp. were immobilized. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Target Species 1  & **Immunosensor Response *** & **Count on Selective Agar (CFU/mL) *** \\ \hline _Salmonella enterica Typhimurium_ & + & \(1.5\times 10^{7}\) \\ CECT4266 04/2023 & + & \(1.5\times 10^{8}\) \\ CECT4266 04/2023 & + & \(5\times 10^{3}\) \\ _Salmonella enterica Typhimurium_ & + & \(5\times 10^{1}\) \\ CECT4266 05/2023 & + & \(5\times 10^{1}\) \\ _Salmonella enterica Typhimurium_ & + & \(5\times 10^{2}\) \\ CECT4266 05/2023 & + & \(1.8\times 10^{10}\) \\ _Salmonella enterica Enteriditis_ & + & \(1.8\times 10^{8}\) \\ CECT 7236 04/2023 & + & \(1.8\times 10^{7}\) \\ _Salmonella enterica Enteriditis_ & – & \(0\) \\ _Salmonella enterica Enteriditis_ & + & \(1\times 10^{2}\) \\ CECT 7236 04/2023 & + & \(1\times 10^{2}\) \\ _Salmonella enterica Enteriditis_ & + & \(1\times 10^{2}\) \\ CECT 7236 05/2023 & + & \(1\times 10^{3}\) \\ _Salmonella enterica Enteriditis_ & – & \(1\times 10^{1}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 8: _Cont._Figure 8: **Calibration strains and food sample curves. ** (**A**) Calibration curve of biosensor against _Salmonella enterica Typhimurium_. (**B**) Calibration curve of biosensor against _Salmonella enterica Enteritidis_. (**C**) Biosensor calibration curve against _Salmonella_ spp. isolated from spiked fresh chicken samples. (**D**) Biosensor calibration curve against _Salmonella_ spp. isolated from spiked red Batavia samples. The analysis of the LoD in chicken samples after using this biosensor were promising. After 3 h of enrichment, the biosensor method achieved a 100% detection rate for samples containing as few as 10 CFU/mL of _Salmonella_ spp. (\(p\) = 0.0026). The biosensor method had a relative accuracy of 100%, indicating that the results from the biosensor fully agreed with those from the reference method when testing identical samples. The sensitivity results were similar. Hence, the biosensor method detected samples contaminated with _Salmonella_ spp., whereas samples without _Salmonella_ spp. were not detected. This finding confirmed the specificity of the biosensor method in distinguishing between true negatives and true positives for the presence of the _Salmonella_ spp. The positive predictive value of the biosensor method was 100%, indicating that it was highly reliable in correctly detecting positive samples.