Multiplex ratiometric gold nanoprobes based on surface-enhanced Raman scattering enable accurate molecular detection and imaging of bladder cancer

Recently, surface-enhanced Raman scattering (SERS) has been successfully used in the non-invasive detection of bladder tumor (BCa). The internal standard method was considered as an effective ratiometric strategy for calibrating signal fluctuation originated from the interference of measurement conditions and samples. However, it is still difficult to detect the target mRNA quantitatively using the current ratiometric SERS nanosensors. In this study, we developed an internal reference based ratiometric SERS assay. Two kinds of molecular beacons (MB) carrying Raman reporter molecules were anchored on sea-urchin-like Au nanoclusters (AuNCs). Thymidine kinase1 (TK1) MBs with hexachlorofluorescein (HEX) were used to capture tumor biomarker TK1 mRNA, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) MBs with 5(6)-carboxyfluorescein (FAM) were used to offer internal standard signals. The internal reference GAPDH MB can reflect the consistent content of the GAPDH mRNA in single cells. The ratiometric method (I₇₄₅/I₆₄₅) can more accurately reflect the content of target mRNA in single cells. The ratiometric nanoprobes had excellent stability (coefficient of variation: 0.3%), high sensitivity (detection limit: 3.4 pM), high specificity (capable of single-base mismatch recognition) and ribozyme-resistant stability. Notably, the nanoprobes can effectively distinguish BCa cells from normal cells, and it was easy to contour the single BCa cell using the ratiometric method. By combining asymmetric polymerase chain reaction (PCR) and ratiometric nanoprobes, it was easy to distinguish the SERS ratio (I₇₄₅/I₆₄₅) as low concentration as 10⁻¹⁴ M. Further clinical detection in urine samples from patients with BCa confirmed its potential for early noninvasive diagnosis of BCa with the sensitivity of 80% and specificity of 100%, which is superior to the current urine cytological method.