The effect of varied pH environment on the optical efficiency of ZnS nanowires and CdSe/ZnS quantum dots as biomarkers

Most biochemical reactions essential for life take place in a variety of buffers in a narrow range of pH values. This paper highlights the importance of media/buffers with different pH values when used for developing nanostructure based biomarkers. Very recently ZnS nanowires and ZnS coated CdSe quantum dots are being employed as selective biomarkers. The optical properties of ZnS-NW and CdSe/ZnS QDs strongly depend on the pH of the media. The work is then extended to the photoluminescence spectroscopy of water soluble CdSe/ZnS core/shell quantum dots in a varied pH of the phosphate buffer saline (PBS) media. Photoluminescence spectroscopy (PL) of mercaptoacetic acid (MAA) and a cardiac antibody (digoxin) conjugated ZnS-NW showed strong dependence of PL intensity on the pH of the media. It demonstrated that the pH and the pKa value of used thiol molecule were critically linked with each other. Similarly, the photoluminescence spectroscopy of the water-soluble CdSe/ZnS dots at varied pH (4 to 14) showed a strong dependence on the pH. The QD emission was found stable when the pH of the solution was close to 7. However, a strong decrease in luminescence intensity was observed in strong acidic and basic conditions, which was attributed to the presence of Lewis acid: protons (H+) and sodium ions (Na+) and Lewis bases: thiolate (S−) and oxygen ions (O−). At high ionic strength there is agglomeration of quantum dots was due to the formation of sodium salt bridge (COONaOOC) complex along with spectral red shift. The current study suggested the best use of water soluble QDs was at pH 7 to 10 for PBS buffer in different biological environments.