Water-Soluble Quantum Dots
Water-soluble quantum dots (QDs) are a relatively novel nanomaterial, one that's already in high-demand in many scientific and industrial settings for its remarkable optical capabilities and usefulness. These nanoparticles are generally made of semiconductors like cadmium selenide (CdSe) or cadmium sulfide (CdS), with core sizes of 2 to 10 nanometres, and their quantum confinement affects their electronic and optical behaviour. And unlike regular quantum dots, water-soluble quantum dots will absorb equally into water, making them a good candidate for biological and environmental research.
Structure and Surface Modification
The core materials of water-soluble quantum dots generally consist of semiconductor materials like CdSe, CdTe, ZnS, or InP. To improve water solubility, the quantum dot surface is often modified with hydrophilic functional groups or biocompatible molecules. Common surface modification methods include:
Hydrophilic Ligands: By altering the quantum dot surface by substituting molecules with hydrophilic groups (carboxyl (-COOH), amino (-NH2) or hydroxyl (-OH) groups) the surface is ultrahydrophilic. Thioglycolic acid, for instance polyethylene glycol (PEG), citric acid and many others are used as surface ligands for better water-phase dispersion and stability.
Polymer Coating: By coating the quantum dots with hydrophilic polymer (chitosan or PEI) it gives quantum dots high stability and biocompatibility.
Biomolecule manipulation: Quantum dots can be atop of biomolecules such as proteins, peptides or DNA to bring them out for bio-analysis.
Prominent Characteristics of Water-Soluble Quantum Dots
Water-soluble quantum dots are very versatile, and they are the best choice for many high-end tasks.
Optical Properties: The quantum dots have very visible fluorescence, high quantum yield, small emission spectra and excellent photostability. Their optical behavior is a function of size, so they can be tuned down to emission wavelength and thus used for multiplexing.
Biocompatibility: Water-soluble quantum dots are hydrophilic on their surface ligands, so they are better water-soluble and biocompatible. And that's why they are perfect for in vivo imaging and biomedical research.
Surface Modification: The surface of water-soluble quantum dots can be modified for applications. Common modifications include PEGylation (polyethylene glycol coating) or functionalisation with biomolecules to allow for precise application and biocompatibility.
Stable Dispersion: This is the biggest advantage of water-soluble quantum dots, as they're stable in water and hence a very useful material for biological and environmental research.
Synthesis Methods of Water-Soluble Quantum Dots
There are various water-soluble quantum dots synthesis methods, but the usual ones are aqueous phases reaction making the quantum dots water and biocompatible. These are some of the usual synthesis techniques:
Direct Synthesis in Aqueous Phase: Use water-soluble thiol stabilizers as stabilisers to directly synthesise quantum dots in water. By way of illustration, stabilizers like mercaptoacetic acid or glutathione enable synthesis of CdTe, CdSe and other quantum dots in solutions. The benefit of this is that it's very easy and cheap to do, although it may require a tuning of reaction parameters for yields and quantum efficiency.
Ligand Exchange Method: This method involves replacing the surface ligands of quantum dots from traditional organic phases with hydrophilic ligands. For instance, oleic acid stabilizing ligands can be replaced with 2-aminoethyl thiol (AET) to dissolve PbSe quantum dots in aqueous media. Also, ligand exchange by glutathione or other bioligands can help quantum dots to be water-soluble.
Microwave Method: By heating the microwaves to produce the water-soluble quantum dots very rapidly. With L-glutathione as reducing agent and n-[3-(trimethoxysilyl)propyl]ethylenediamine as silicon source, for instance, fluorescent silicon quantum dots can be produced in the microwave at home. This makes the experimental work easier and the reaction time shorter.
Core-Shell Structure Technique: It provides water absorption by securing a shell over the quantum dots. For example, CdSe/ZnS core-shell quantum dots can be prepared by coating the CdSe quantum dots' surface with ZnS shell so that the surface is clean and for fluorescence to be quality and stable.
Hydrothermal Method: Quantum dots are made with solvent, water, and in high temperature/pressure. For example, with ligands like glutathione and sodium citrate, AgInS2 quantum dots can be produced hydrothermally. This method effectively passivates surface defects and improves quantum yield.
Application Fields
Water-soluble quantum dots have broad and diverse applications across multiple sectors due to their versatile optical properties and ability to be engineered for specific tasks.
Biomedical Applications
Bioimaging: Water-soluble quantum dots are the most common quantum dots in biological imaging with high luminosity, narrow emission spectra and long fluorescence lifetimes. You can imaging cells and tissues with them for molecular-scale imaging.
Drug Delivery: Quantum dots can be targeted with antibodies or peptides to deliver drugs into cells or tissues. They're even small enough to penetrate biological defences and provide targeted drug delivery.
Biosensors: Water-soluble quantum dots can be used for biosensors of proteins, nucleic acids and small molecules. Their fluorescence response to environment can be extremely sensitive and rapid.
Environmental Monitoring
Pollution Monitoring: Water-soluble quantum dots are highly sensitive to pollutants present in the environment like heavy metals, pesticides, etc. They can be inserted in water quality monitors or to identify harmful chemicals in the environment.
Purification in Water: Due to the surface area and flexibility of quantum dots, quantum dots can be used in water purification systems to clean contaminants or pathogens.
Optoelectronics
Light-Emitting Devices: Quantum dots are being added to light emitting devices (LEDs, display). They have tunable emission that allows for applications such as quantum dots displays (QLEDs) for bright energy-efficient displays.
Solar Cells: Water-soluble quantum dots with high quantum efficiency and good transport of charge are also being studied for photovoltaic panels that increase the conversion efficiency of solar energy.
References
- Wu J, Li J, Cheng M, et al. Water-soluble near-infrared AgInS2 quantum dots for Ca2+ detection and bioimaging[J]. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2024, 322: 124859.
- Sharma K, Raizada P, Hasija V, et al. ZnS-based quantum dots as photocatalysts for water purification[J]. Journal of Water Process Engineering, 2021, 43: 102217.
- Kamat P V. Quantum dot solar cells. The next big thing in photovoltaics[J]. The journal of physical chemistry letters, 2013, 4(6): 908-918.
