**4. Conclusions**

In the current study, several LFIA approaches using the antibiotic LIN as the relevant contaminant of food products were performed and compared, including conventional AuNP-based LFIA, fluorescent QD-based LFIA, and SERS-based LFIA. AuNP- and QD-based LFIAs are confined to the limit of detection of the nanodispersed label used and the analyses were carried out in the direct competitive format with use of anti-LIN antibodies labeled with AuNPs or QDs. The colorimetric AuNPs-based LFIA was characterized by the detection limit of 0.4 ng/mL. The replacement of the colorimetric marker with a fluorescent one resulted in a slight enhancement in sensitivity (the detection limit was 0.2 ng/mL). To address current challenges of LFIA biosensors associated with the lack of sensitivity and limits in quantitative analysis, the novel SERS-based LFIA for LIN was developed. The limit of detection determined by SERS experiments was 1.4 × 10−<sup>6</sup> ng/mL. Notably, the sensitivity of AuNP- and QD-based LFIAs are defined by the detection limit of the nanodispersed marker on the test strip, while in the case of SERS-based LFIA an indirect registration of the signal from the Raman reporter molecule using a highly sensitive device is performed. Therefore, the ordinary comparison of the detection limits achieved using the considered three approaches is not quite legitimate and the choice of a nanodispersed marker and a signal detection technique should be determined by several parameters, in particular, the aim of the study, facilities of the laboratory, the nature of the target analyte and requirements to its maximum residue limits. The proposed SERS-based LFIA, which possesses both high sensitivity and quantitative evaluation capabilities, confirms the effectiveness of the SERS technique for the sensitive detection of target analytes. This implies that handheld Raman readers for quantitative LFIA could potentially facilitate sensitive point-of-care tests. To our knowledge, ours is the first report of quantitative LIN detection by fluorescence and SERS-based LFIA, which also presents a promising tool for other contaminants.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-6374/10/12/198/s1. Figure S1: Competitive curve for the ELISA of LIN. The detection limit of LIN (IC10) is 0.08 ng/mL, its concentration causing 50% inhibition of the antibody binding (IC50) is 0.69 ng/mL. The error bars indicate the standard deviations for three measurements.

**Author Contributions:** Conceptualization, B.B.D.; methodology, A.V.Z.; formal analysis, K.V.S., O.D.H. and E.A.Z.; investigation, K.V.S., O.D.H., E.A.Z. and D.S.P.; resources, C.X.; writing—original draft preparation, K.V.S.; writing—review and editing, O.D.H. and A.V.Z.; visualization, K.V.S., O.D.H. and E.A.Z.; supervision, B.B.D.; project administration, A.V.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the Russian Science Foundation (Project 19-14-00370).

**Acknowledgments:** The authors are grateful to S.M. Pridvorova (Research Center of Biotechnology of the Russian Academy of Sciences) for obtaining electronic microphotographs of the AuNPs, and to D.V. Kuznetsov (National University of Science and Technology (MISiS), Moscow, Russia) for assistance in using the DXR Raman Microscope.

**Conflicts of Interest:** The authors declare no conflict of interest.
