Quantum dots (QDs) have unique optical properties such as high quantum yield, broadband absorption, narrowband and tunable emission, good anti-photobleaching, electronic properties of charge/energy transfer, and chemical properties of easy surface functionalization and good pH stability. Owing to their unique characteristics, the QDs have been widely developed as fluorescent sensors with great sensitivity. In this thesis, the QD-molecule system and QD-aptamer/GO system were, respectively, constructed by metal coordination and bioconjugation, and used as fluorescent sensors for ion, pH and biomolecule sensing. The developed QDs-based fluorescent sensors have the advantages regarding high sensitivity, high specificity and/or logic diversity. The main contents are as follows:
1.Multi-colored and multi-structured QDs were synthesized. Water-soluble TGA-capped CdTe QDs were synthesized according to the “one pot” method with CdCl2, TGA and NaHTe as raw materials. InP/ZnS core/shell QDs were synthesized following the “successive ion layer adsorption and reaction (SILAR)” method. The as-prepared QDs exhibit high fluorescence quantum yields, narrow full-width-at-half-maximum (FWHM), and size (composition-)-tunable emission (495 nm-602 nm).
2.The “QD-molecule”-configurated fluorescent sensor was constructed with CdTe QD as the scaffold and 1, 10-phenanthroline (Phen) as the ligand, and a novel ligand displacement-induced fluorescence switch strategy was developed for highly sensitive and selective detection of Cd2+. The metal affinity-driven complexation of Phen on CdTe QD quenches the QD emission by a photoinduced hole transfer (PHT) mechanism. In the presence of Cd2+, the Phen ligands are readily detached from CdTe QD due to higher affinity toward the analyte, interrupting the PHT process. As a consequence the fluorescence of CdTe QD switches on. The limit of detection (LOD) for Cd2+ detection was estimated to be ~0.01 nM, and the linear dynamic range (LDR) was defined as 0.02 nM to 0.6 μM. Importantly, this QD-Phen sensor featured with high specificity, being able to discriminate Cd2+ with Zn2+.
3.By using dual-colored InP/ZnS QDs to label two different aptamers (one for thrombin (Thr) and another for adenosine triphosphate (ATP)) (TBA and ABA), respectively, combining with graphene oxide (GO), two “QD-aptamer/GO”-configurated fluorescent sensors (GO/TBA-QD506 and GO/ABA-QD571) were constructed, and developed for pH sensing by utilizing structural tunability of aptamers. By chaning pH, the sensors in the presence of Thr or ATP exhibited reversible “S”-shaped fluorescence titration curves with the sharp transition in the range from pH 6.0-7.5. At pH > 8.0, the QD506 or QD571 showed high fluorescence, while the low fluorescence of both QD506 and QD571 were observed at pH < 6.0. The sensing mechanism relies on long range resonance energy transfer (LrRET). At basic pH, TBA bind with Thr to form G-quadruplex, and ABA bind with ATP to form duplex, releasing TBA-QD506 and ABA-QD571, respectively, from GO and interrupting the LrRET. At acidic pH, the G-quadruplex or duplex is denatured to single strands, re-adsorbing of TBA-QD506 and ABA-QD571 on GO and quenching the QDs emission by LrRET. Furthermore, benefitting from the strong anti-photobleaching properties of QDs, reliable fluorescent switches were demonstrated by utilizing the fluorescent pH-depedence of the sensors.
4.The GO/TBA-QD506/ABA-QD571 system was constructed by silmutaneous deposition of TBA-QD506 and ABA-QD571 on GO, and developed to implement logic circuits including half-adder and half-subtractor. A half adder operation was implemented by using ATP and Thr as two inputs. In the initial state, TBA-QD506 and ABA-QD571 were adsorbed onto GO. In the presence of either input, the specific recognition interaction of TBA-Thr or ABA-ATP folded the respective aptamer to G-quadruplex or duplex, releasing TBA-QD506 and ABA-QD571 from GO, respectively. In the presence of two inputs, both of TBA-QD506 and ABA-QD571 were desorbed from GO. The adsorption/desorption of QD-labelled aptamers led to fluorescent response of the system, which fits the characteristics of half adder. The system was readily reconfigured to implement a half subtractor operation by using Thr coexisted with DNA1 (label-free ATP binding aptamer), and ATP coexisted with DNA2 (label-free Thr binding aptamer) as the two inputs. Either TBA-QD506 or ABA-QD571 was desorbed from GO in the presence of either input, owing to the specific TBA-Thr or ABA-ATP recognition interaction, not interfered by the coexisted DNA1 or DNA2. In the presence of both inputs, however, the DNA1-ATP and DNA2-Thr recognition interactions occurred in priority, inhibiting the TBA-Thr and ABA-ATP interactions, and the system remained its initial state. Accordingly, the high fluorescence of either QD506 or QD571 was observed in the presence of either input, while in the presence of none or both of the inputs, the fluorescence of both QD506 and QD571 remained low. This fluorescent response fits the characteristics of half subtractor. Furthermore, repetitious half-adder and half-subtractor arithmetic operations were achieved benefitting from fluorescence stability of the QDs. In addition, the logic system can be used as fluorescent sensors for detection of Thr and ATP. The obtained LDRs for ATP and Thr were 0.1 μM to 30 μM and 0.1 nM to 50 nM, respectively; the LODs for ATP and thrombin were estimated to be 40 nM and 50 pM, respectively.