Purpose: Candidemia infection is common in the clinic and has a high mortality rate. Candida albicans, Candida tropicalis , and Candida krusei  are very important and common pathogenic species. Candida  is difficult to isolate from clinical samples and culture, and immunological detection cannot distinguish these related strains. Furthermore, Candida  has a complex cell wall, which causes difficulties in the extraction of DNA for nucleic acid detection. The purpose of this study was to establish a protocol for the direct identification of Candida  from serum.
Materials and Methods: We synthesized Fe₃O₄@PEI (where PEI stands for polyethylenimine) magnetic nanoparticles to capture Candida  and prepared positively charged silver nanoparticles (AgNPs⁺) as the substrate for surface-enhanced Raman scattering (SERS). Candida  was directly identified from serum by SERS detection.
Results: Orthogonal partial least squares discriminant analysis (OPLS-DA) was used as the multivariate analysis tool. Principal component analysis confirmed that this method can clearly distinguish common Candida . After 10-fold cross-validation, the accuracy of training data in this model was 100% and the accuracy of test data was 99.8%, indicating that the model has good classification ability.
Conclusion: The detection could be completed within 40 minutes using Fe₃O₄@PEI and AgNPs⁺ prepared in advance. This is the first time that Fe₃O₄@PEI was used in the detection of Candida  by SERS. We report the first rapid method to identify fungi directly from serum without breaking the cell wall to extract DNA from the fungi.
Keywords: capture, surface-enhanced Raman scattering, positively charged silver nanoparticles, orthogonal partial least squares discriminant analysis, 10-fold cross-validation