Fast detection of Staphylococcus aureus using thiol-functionalized WS2 quantum dots and Bi2O2Se nanosheets hybrid through a fluorescence recovery mechanism.

Ultrafast and sensitive detection of Staphylococcus aureus (S. aureus), a harmful Gram-positive human pathogenic bacterium, by two-dimensional layered materials continues to be a challenge. Herein, we have studied the sensing of S. aureus using a tungsten disulfide (WS2) quantum dot (QD) and bismuth oxyselenide (Bi2O2Se) nanosheet (NS) hybrid through their unique optical functionalities. The WS2 QDs of a mean diameter of 2.5 nm were synthesized by liquid exfoliation. Due to the quantum confinement and functional groups, the WS2 QDs exhibit high fluorescence (FL) yield under UV excitation. The addition of Bi2O2Se NSs resulted in the adsorption of WS2 QDs on their surface, resulting in quenching of the FL emission due to nonfluorescent complex formation between the WS2 QDs and Bi2O2Se NSs. A specific sequencing single-standard DNA (ssDNA) aptamer, which identifies and explicitly binds with S. aureus, was attached to the defect sites of the WS2 QDs for selective detection. The thiol-modified ssDNA aptamers attach covalently to the WS2 QD defect sites, which was confirmed by Raman and X-ray photoelectron spectroscopy (XPS). The interaction of S. aureus with the aptamer functionalized WS2 QDs weakens the van der Waals interaction between the WS2 QDs and Bi2O2Se NSs, which results in the detachment of the WS2 QDs from the Bi2O2Se NS surface and restores the FL intensity of the WS2 QDs, thus allowing the efficient detection of S. aureus. Similar measurements with non-targeted bacteria show that the system is quite selective towards S. aureus. Our FL-based biosensor has a linear response in the range of 103-107 CFU mL-1 (colony formation unit mL-1) with a detection limit of 580 CFU mL-1. We have observed a fast response time of 15 minutes for sensing, which is superior to the previous reports. The proposed system was tested in human urine and can detect S. aureus in human urine samples selectively, proving its potential in real-life applications. The reported approach is versatile enough for sensing other biomolecules and metal ions by choosing suitable receptors.

Two-dimensional bismuth oxyselenide quantum dots as nanosensors for selective metal ion detection over a wide dynamic range: sensing mechanism and selectivity.

Bismuth oxyselenide (Bi2O2Se) nanosheets, a new 2D non-van der Waals nanomaterial having unique semiconducting properties, could be favorable for various sensing applications. In the present report, a top-down chemical approach was adopted to synthesize ultrathin Bi2O2Se quantum dots (QDs) in an appropriate solution. The as-prepared 2D Bi2O2Se QDs with an average size of ∼3 nm, exhibiting strong visible fluorescence, were utilized for heavy-metal ion detection with high selectivity. The QDs show a high optical band gap and a reasonably high fluorescence quantum yield (∼4%) in the green region without any functionalization. A series of heavy metal ions were detected using these QDs. The as-prepared QDs exhibit selective detection of Fe3+ over a wide dynamic range with a high quenching ratio and a low detection limit (<0.5 µM). The mechanism of visible fluorescence and Fe3+ ion-induced quenching was investigated in detail based on a model involving adsorption and charge transfer. Density functional theory (DFT) first principles calculations show that fluorescence quenching occurred selectively due to the efficient trapping of electrons in the bandgap states created by the Fe atoms. This work presents a sustainable and scalable method to synthesize 2D Bi2O2Se QDs for heavy metal ion sensing over a wide dynamic range and these 2D QDs could find potential uses in gas sensors, biosensors and optoelectronics.

Room temperature exciton formation and robust optical properties of CVD-grown ultrathin Bi2O2Se crystals on arbitrary substrates.

The appealing success of non-van der Waals (non-VdW) two-dimensional (2D) bismuth oxyselenide (Bi2O2Se) crystals in optoelectronics provides an exciting avenue to investigate their fundamental physical properties. To date, the majority of efforts have focused on understanding the properties of 2D Bi2O2Se, usually grown on a mica substrate. However, a gap exists in realizing the origin of photoluminescence (PL) of new age non-VdW Bi2O2Se at visible and near-infrared (NIR) wavelengths and the effect of growth substrates on the structure and optical properties. Herein, we report that the formation of multiple excitons in momentum valleys is responsible for broadband absorption and visible PL from a few layer thick 2D Bi2O2Se. The effect of growth substrates on the structure and optical properties is investigated in detail. Our studies unfold that the growth substrates (mica, sapphire, quartz, SiO2, glass) introduce strain/doping in chemical vapor deposition (CVD)-grown Bi2O2Se crystals, and consequently, the morphology, lattice constant, absorption coefficient, optical bandgap, refractive index, and PL properties are modulated. In addition, the possible direct/indirect multiple exciton formation at the valence band to the conduction band at different symmetry points of Bi2O2Se is analyzed from experimental data on different growth substrates and corroborated with the density functional theory (DFT) calculation of the electronic band structure. Furthermore, temperature-dependent photo-carrier dynamics discloses an A/Γ-exciton activation energy of 209.6 meV in Bi2O2Se. These findings are significant for the futuristic optoelectronic applications of Bi2O2Se and the choice of growth substrates on directly fabricated nanodevices.

Understanding the interfacial charge transfer in the CVD grown Bi2O2Se/CsPbBr3 nanocrystal heterostructure and its exploitation in superior photodetection: experiment vs. theory.

Efficient charge transfer in a 2D semiconductor heterostructure plays a crucial role in high-performance photodetectors and energy harvesting devices. Non-van der Waals 2D Bi2O2Se has enormous potential for high-performance optoelectronics, though very little is known about the interfacial charge transport at the corresponding 2D heterojunction. Herein, we report a combined experimental and theoretical investigation of interfacial charge transfer in the Bi2O2Se/CsPbBr3 heterostructure through various microscopic and spectroscopic tools corroborated with density functional theory calculations. The CVD-grown few-layer Bi2O2Se nanosheet possesses high crystallinity and a high absorption coefficient in the visible-near IR region. We integrated the few-layer Bi2O2Se nanosheet possessing superior electron mobility and CsPbBr3 nanocrystals with high light-harvesting capability for efficient broadband photodetection. The band alignment reveals a type-I heterojunction, and the device under reverse bias reveals a fast response time of 12 µs/24 µs (rise time/fall time) and an improved responsivity in the 390 to 840 nm range due to the effective interfacial charge transfer and efficient interlayer coupling at the Bi2O2Se/CsPbBr3 interface. Notably, a photodetector with a better light on/off ratio and a peak responsivity of ∼103 A W-1 was achieved in the Bi2O2Se/CsPbBr3 heterostructure due to the synergistic effects in the heterostructure under ambient conditions. The DFT analysis of the density of states and charge density plots in the heterostructure revealed a net transfer of electrons/holes from perovskite nanocrystals to Bi2O2Se layers and additional density of states in Bi2O2Se. These results are significant for the development of non-van der Waals heterostructure based high-performance low-powered photodetectors.