In recent decades, quantum dots, also known as semiconductor nanocrystals, have become the focus of attention due to their unique electronic and luminescent properties and their applications in biomarkers, light-emitting diodes, lasers and solar cells. Compared to traditional organic dyes, quantum dots have excellent luminescence properties, such as adjustable fluorescence emission, narrow and symmetrical emission spectra, continuous absorption spectra, and light stability. Quantum dots of different emission wavelengths can be obtained by adjusting the size of quantum dots. The narrow and symmetrical fluorescence emission makes quantum dots an ideal material for multi-color marking.

Since quantum dots have the above advantages, it is desirable to prepare stable quantum dots having a broad absorption band, a high quantum yield, and good biocompatibility. At present, there are many methods for preparing quantum dots, including colloid method, chemical etching method, organometallic method, hydrothermal method and the like.

Colloidmethod

The colloidal method refers to a method in which an organic or inorganic substance of the metal solidifies is solidified in a solution or sol to form a quantum dot, which can be applied to the surface of the substrate by centrifugal force and annealed to form a desired nanocluster. This method attaches the colloid to the nanopore template. After annealing process in high temperature, the template is automatically removed, and the colloidal particles form crystals at high temperature. The size of the particles is related to the size of the template pores, the colloidal concentration and the annealing temperature. This method can produce quantum dots on a large scale without complicated equipment, and it is simple in and low in cost. The disadvantage of this method is that it is difficult to form high quality crystal particles and is highly susceptible to dust contamination in the air.

Chemical etching method

Chemical etching method means that the quantum dot material is directly obtained by etching the crystal substrate. Due to the difference in the etching rate of the chemical liquid to different semiconductor materials and the anisotropy of the crystalline material, the pattern can be left on the crystal substrate by etching lithography to obtain nanometer-scale quantum dots. The quantum dots prepared by this method exhibit high purity and superior performance, but the size of the quantum dot materials is very inconsistent due to the anisotropy of corrosion.

Organometallic method

The organometallic method is the main method used to prepare quantum dots in organic systems. The organometallic method refers to a method for preparing quantum dots by pyrolysis of an organometallic precursor in an organic solvent of high boiling point. In this method, the organometallic precursor solution is injected into a ligand solution at 250℃ to 300 ℃, and the precursor is rapidly pyrolyzed under high temperature conditions to form a crystal nucleus which slowly grows into a nanocrystal. The precursors used in the ligand are mainly alkyl metals and alkyl non-metals (such as[Bis(trimethylsilyl)]selenide) compounds. The advantage of this method is that a large number of quantum dots can be prepared and the yield of quantum dots is high.

Hydrothermal method

The hydrothermal method is a combination of inorganic synthesis and crystal preparation. In a closed reactor (such as a high pressure reactor), water is used as a reaction system for inorganic synthesis and material preparation by heating water to a supercritical temperature or near a supercritical temperature to generate a high pressure in the reaction system. Due to the increase in synthesis temperature, the period of production of quantum dots is significantly shortened. Due to the separation of nucleation and growth processes, the surface defects of quantum dots have been greatly improved, and the yield of fluorescent quantum has also been significantly increased.Therefore, the hydrothermal method has become the main synthesis method for quantum dots directly used as bioluminescent probes.