![]() 4, 5, 6 In particular, the bioconjugation of peptides or proteins to CdSe molecular clusters is very promising for fluorescent probes. 1, 2, 3 The coupling of organic dyes, proteins or DNA to CdSe nanoclusters has produced nanomaterials with sensing, electroluminescent, and therapeutic applications. Modern colloidal preparation methods allow fine control of the optical and electronic properties of II-VI semiconductor nanoparticles. The results suggest that the − adsorption induces a redshift in the excitation energies in a surfactant environment. Surface passivation of (CdSe) 6 − is then simulated by coating it with multiple NH 3 molecules. As far as NH 3 is a faithful representation of a surfactant, the results clearly illustrate the differences between the electronic effects of an alkylthiolate versus those of surfactant molecules. As expected, the excitation energies of the passivated (CdSe) 6⋅6NH 3 are also blueshifted compared to (CdSe) 6. This has the effect of widening the HOMO-LUMO gap of (CdSe) 6⋅6NH 3 compared to (CdSe) 6. In contrast, binding of multiple NH 3 destabilizes cadmium 5 p orbitals, which contribute significantly to the lowest unoccupied molecular orbital (LUMO) of (CdSe) 6, while leaving the selenium 4 p orbitals near the HOMO relatively unaffected. The resulting smaller HOMO-LUMO gap of (CdSe) 6 − indeed leads to redshifts in its excitation energies compared to (CdSe) 6. This difference shows that − binding effectively introduces filled sulfur orbitals above the selenium 4 p orbitals of (CdSe) 6. The highest occupied molecular orbital (HOMO) of (CdSe) 6 is dominated by selenium 4 p orbitals in contrast, the HOMO of (CdSe) 6 − is dominated by sulfur 3 p orbitals. Natural bond orbital results show that the redshift is caused by ligand-inorganic cluster orbital interaction. This study examines this issue by performing density functional theory (DFT) and time-dependent-DFT calculations to study the ground state and low-lying excited states of (CdSe) 6 −, a transition metal complex built by binding methanethiolate to a CdSe molecular cluster. Experimentally, the binding of a short peptide has been found to redshift the optical absorption of nanoclusters. Bioconjugated CdSe quantum dots are promising reagents for bioimaging applications.
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