Highly fluorescent and robust semiconductor nanocrystals (known as quantum dots or QDs) play a pivotal role in biological applications. In particular, the excellent optical properties of QDs make them more suitable for immunolabeling, molecular imaging, and multiplexed biological detection for cellular analysis than conventional fluorescent dyes. Many studies have applied QD probes for in vitro and in vivo assays, showing great improvements with respect to gaining insight into bioanalytical chemistry, target specificity, and cytotoxicity. In this review, we discuss the optical properties, specificity, and cytotoxic effects of QDs as well as the progress achieved in multicolor cellular imaging, immunolabeling for signaling pathways, and molecular detection at the cellular level. In addition, carbon QDs as alternatives to the toxic cadmium-based QDs and their applications in biotechnology are discussed. Despite the rapid development and recent progress of QD probes, much more work is required to determine the toxicity of cadmium-containing QDs used in live cells and animals.

Quantum dot (QD) multicolor analysis was achieved by labeling the epidermal growth factor and labeling the antibody against the epidermal growth factor receptor (anti-EGFR) using different QDs. On the basis of the fluorescence intensity of the two types of QDs, the amount of epidermal growth factors (EGFs) in the cells and anti-EGFRs bound to EGFRs were analyzed. The functions of anti-EGFR in preventing HeLa cells from engulfing EGF and inhibiting overproliferation of the HeLa cells were also studied. Meanwhile, parallel analysis was conducted to analyze the heterogeneity of cells at the single-cell level using a single-cell array. This provides a novel approach for the multiplexed analysis for the anti-EGFR functions in cells together with the cell heterogeneity. It also lays a foundation for parallel analysis and detection, using various fluorescence probes, simultaneous tracking, and detection of multiple targets at an overall level or individual level, and multichannel analysis of drug effects.

Conventional probes used to detect proteases are susceptible to either photobleaching or low efficiency in cell penetration. Also it is difficult to achieve multiplexed detection. Based on QD, nanogold and EGF, a unique probe was studied in this paper. The sensing section of the probe was developed by linking the streptavidin-labeled QD and monomaleimide-functionalized nanogold via a substrate peptide. The QD fluorescence is partially quenched by the nanogold when they are connected. After the substrate peptide is cleaved by protease, the QD fluorescence can be partially recovered as the distance between two kinds of nanoparticles increases. Biotin-labeled EGF was used to carry the sensing section into the cells, making the transfection efficiency higher than former nanoprobes. After assays, we found the optimal ratios of nanogold to QD and to EGF to realize the high efficiency of both quenching and transfer. Also an algorithm was proposed to evaluate the relative activity of proteases. Finally, caspase-3 was used as the target protease to be detected. The activity of caspase-3 was successfully monitored during a longer time span compared to the reported probes.

Bio-MEMS technique of organizing cells in single cell arrays makes it easier to observe cells’ individual characteristics and behaviors, which is of benefit for basic cell research and high throughout drug screening. We utilized photolithography and chemical vapor deposition (CVD) to pattern hydrophobic hexamethyldisilazane (HMDS) islands and hydrophilic polyethylene glycol (PEG)-Silane regions on 25 mm × 25 mm glass slides. Compared with methods presently used, the photoresist wells can be more stable, and clear HMDS island arrays can be formed. We adopted a better and more powerful medium, the Biotin–(Strept)Avidin System to fix cells on the specified regions of the substrate. This is more efficient than the former medium, antibodies and antigens. Moreover, using a biotinylating cell surface, we produced more biotins on the surface of the cells and made it easier to capture cells and avoid washing away fixed cells. By changing the concentration of cell suspension for seeding, we found a suitable concentration (5 × 106 cells/ml), at which the cell occupation was greater than 90%. By comparing various diameters of streptavidin islands, optimal diameters (14–20 μm) were found to capture single human promyelocytic leukemia cells (HL-60). With all optimal parameters, single cell arrays were formed. The ratio of islands capturing only one cell was approximately 77%, which is better than similar approaches.