Multiple sensor systems are designed by varying aza-crown ether moiety in silicon quantum dots (SiQDs) for detecting individual Mg(2+), Ca(2+), and Mn(2+) metal ions with significant selectivity and sensitivity. The detection limit of Mg(2+), Ca(2+), and Mn(2+) can reach 1.81, 3.15, and 0.47 μM, respectively. Upon excitation of the SiQDs which are coordinated with aza-crown ethers, the photoinduced electron transfer (PET) takes place from aza-crown ether moiety to the valence band of SiQDs core such that the reduced probability of electron-hole recombination may diminish the subsequent fluorescence. The fluorescence suppression caused by such PET effect will be relieved after selective metal ion is added. The charge-electron binding force between the metal ion and aza-crown ether hinders the PET and thereby restores the fluorescence of SiQDs. The design of sensor system is based on the fluorescence "turn-on" of SiQDs while in search of the appropriate metal ion. For practical application, the sensing capabilities of metal ions in the live cells are performed and the confocal image results reveal their promising applicability as an effective and nontoxic metal ion sensor.

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Several distinct surface-functionalized molybdenum diselenide (MoSe 2 ) quantum dots (QDs) were developed as chemosensors based on the fluorescent probe. The carboxylic-, amine-, and thiol-functionalized MoSe 2 QDs (MoSe 2 /COOH, MoSe 2 /NH 2 , and MoSe 2 /SH) were synthesized by tuning their surface with thiol-containing capping agents. These MoSe 2 / COOH and MoSe 2 /NH 2 QD sensors were implemented for the highly selective and sensitive detection of copper ion (Cu 2+ ) and 2,4,6-trinitrophenol (TNP) with a lower detection limit of 4.6 and 45.3 nM, respectively. Similarly, the MoSe 2 /SH QDs while coupled with gold nanoparticles showed excellent selectivity toward melamine (MA) with a lower detection limit of 27.7 nM. It is surprising to find that each functionalized QD exhibits a distinct sensing mechanism in the detection of Cu 2+ , TNP, and MA, based on metal-ion-induced fluorescence turn-on, electron transfer, and energy transfer suppression, respectively. Moreover, these MoSe 2 QD-based chemosensors were successfully utilized in real samples, confirming their propitious application.

Herein, we report ultra-sensitive sensing of a prostate-specific antigen (PSA), which is used as a biomarker to detect prostate cancer, using a molybdenum series (MoO, MoS, and MoSe) of two-dimensional nanosheets (2D NSs). Moreover, the design of a 2D NS-based PSA aptamer sensor system was demonstrated based on a fluorescence turn-on mechanism in the presence of a target. The 2D NSs acted as an excellent sensing platform in which the PSA aptamer was adsorbed on the NSs and subsequent energy transfer between them led to fluorescence quenching of the aptamer. The detection limit of PSA was achieved to be 13 pM for MoO NSs, whereas the MoS and MoSe systems exhibited a detection limit of 72 and 157 pM, respectively. To the best of our knowledge, this is the first report on the ultra-sensitive detection of a 2D NS-based aptamer sensor. The in vitro bioimaging measurements were performed using confocal fluorescence microscopy. Herein, PSA detection was successfully demonstrated in human embryonic kidney 293T (HEK) live cells. Moreover, the MoO, MoS, and MoSe NSs exhibit excellent biocompatibility and low toxicity; thus, these 2D NSs can be used as a promising sensor platform to detect prostate cancer.