7.3.8. Vitamins

Vitamins, which are found in many animal and plant tissues, play an essential role in proper metabolism and maintenance of body cells. Disorders in their synthesis or metabolism may be the cause of many serious diseases, hence the invention of sensitive and fast sensor devices is significant [112–116]. An exemplary QDs-based sensors for determination of vitamins are presented in Table 10.


**Table 10.** QDs-based sensors for vitamins determination.

Liu et al. [112] prepared a novel optosensing material based on quantum dots and graphene oxide for specific determination of Vitamin E. Ultra-high specific surface was obtained by synthesis of molecular imprinted polymer (MIP), which was stocked for specific Vitamin E reaction area. Under optimal condition, the fluorescence intensity of MIP was decreased linearly with the increasing concentration of Vitamin E. Such constructed sensor exhibited good linear range from 2.30 × 10−2– 9.20 × 10<sup>2</sup> μM with a LOD 3.5 nM.

Another strategy was used by Geszke–Moritz et al. [113] They used high fluorescence sensitivity to folic acid due to the high affinity of nitrogen atoms and carboxyl groups to doped QDs. Due to the quenching of fluorescence intensity QDs, it is possible to detect folic acid concentrations from LOD 11 μM.

Ganiga and others presented an optical sensor that uses FRET for fast and sensitive detection of ascorbic acid (AA). For this purpose, CdS QDs and diphenylcarbadiazone (DPCD) were used. In the presence of AA, the DPCD was transformed into diphenylcarbazide (DPC), which resulted in the recovery of fluorescence. Changes in fluorescence intensity enabled the detection and determination of AA concentration in the linear range of 60–300 nM with LOD 2 nM [116].

#### *7.4. Detection of Bacteria and Viruses*

The identification of pathogenic bacteria and viruses in food, water, air, and body fluids is extremely important because of their drastic impact on our society. The result of the human body's contact with pathogenic bacteria or viruses is serious gastrointestinal infections that can lead to patient death without a doctor's control. Importantly, pathogenic bacteria also produce toxins that are responsible for the occurrence of serious diseases, such as hemorrhagic colitis, characterized by painful abdominal cramps and bloody diarrhea or hemolytic-uremic syndrome, the most severe effect of which is an acute renal failure. Fast and sensitive detection of pathogenic bacteria and viruses is necessary to prevent the occurrence of epidemics or severe forms of the disease [117–121]. Table 11 shows an exemplary strategies for bacteria and viruses determination.



Xue et al. [120] presented a novel fluorescent biosensor for ultra-sensitive and rapid detection of *E.coli* O157:H7 with LOD 14 CFU/mL. The proposed fluorescent biosensor used the double-layer channel with the immune magnetic nanoparticles (MNPs) for specific separation and efficient concentration of the target bacteria, and the immune CdSe/ZnS QDs with a portable optical system for quantitative detection of the bacteria. Initially, the bacteria were captured by the immune MNPs in the channel at the presence of the high gradient magnetic fields (HGMFs) to form the MNP-bacteria complexes. Then, the immune QDs were used to react with the target bacteria to form the MNP-bacteria-QDs complexes in the channel. Finally, the enriched complexes were collected and detected using the portable optical system to obtain increase the fluorescence intensity for final

determination of the *E.coli* O157:H7 cells in the sample. Wu et al. [118] has prepared modified ZnSe/ZnS QDs by 3-mercaptopropionic acid and established a rapid fluorescence method to detect the *E. coli* cells count by using MPA-ZnSe/ZnS QDs as a fluorescence probe. The fluorescence peak intensity increases with increasing cells count of bacteria. Compared with the traditional fluorescent detection methods, this one is more convenient and useful in the bacterial count determination with LOD 10<sup>1</sup> CFU·mL−1.

In addition to bacteria, QDs are also used for virus detection. Jimenez et al. reported work which was focused on the development of a nano-system for simultaneous identification of HIV and HPV viruses with 1 nM of LOD. Their construction and characterization were carried doubt using magnetic glass particles (MGPs) which joined with target DNA oligonucleotides and the second part of the construction formed by the conjugation of red and green CdTe QDs with oligodeoxyribonucleotides complementary probe, derived from these two viruses, that encode respectively their capsid and oncoproteins. As a result, after the conjugating, the fluorescent intensity was slightly reduced in both cases [119].

#### **8. Conclusions and Perspectives**

Quantum dots have remarkable optical properties, which make them among the most useful nanomaterials [6]. They may be utilized in a wide range of applications, e.g., in new types of fluorescent probes and as active components of nanostructure-biomolecule complexes [122]. Various schemes for the application of optical transduction QDs have been successfully tested, allowing a wide range of detection, high selectivity and sensitivity in the tested samples. The development of analytical methods for the detection of various chemical or biological compounds allows the use of QDs in sensors for determining the presence of ions, molecules and pH changes. The results of discussed studies lead to the improvement of existing detection devices and the design of new detection devices that allow more sensitive and faster analysis. Quantum dots-based detection technologies can be adapted to precision medical technologies by overturning point-of-care (POC) and personalized diagnostics. This engineering can supply high-throughput and mobile diagnostic platforms for screening pathogens and toxins immediately in field and POC clinical settings. Several of these technologies tender multiplexing capacities for simultaneous examination of multiple analytes with unexpectedly high sensitivity that can notably lower costs and detection time. Nevertheless, a universal sensor for different types of medicinal/or, i.e., food samples, is a challenge because of the inherent complexity of biological samples. Evaluation with numerous of biological (i.e., food, body fluids) samples and comparison with well-established techniques may assist to direct this challenge.

Among further fields that could exploit some of the advantages of QDs are fluorescent immunosensors designed as integrated devices. Due to the relevant improvement on the execution of fluorescent immunosensors and recent advances in miniaturization processes, it is believed that in the near future small and advanced fluorescent mobile analytical platforms, which combine steps of the immunoassay pathway, will be available. In conclusion, there is potential for further investigations of QDs in multiplex detection, particularly via continued miniaturization and integration into lab-on-chip platforms.

**Author Contributions:** Conceptualization, A.P. and J.C.; Formal Analysis, A.P., J.C. and K.M.; Investigation, A.L. and K.D.; Resources, A.L. and K.D.; Writing-Original Draft Preparation, A.L. and K.D.; Writing-Review & Editing, A.L. and K.D.; Visualization, A.L., A.P. and K.D.; Supervision, J.C., A.P., M.B. and K.M.; Project Administration, A.P., J.C., K.M.; Funding Acquisition, J.C. and K.M.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors gratefully acknowledge the financial support from Wroclaw University of Science and Technology (statutory activity no. 0401/0145/18 and 0401/0137/18).

**Conflicts of Interest:** The authors declare no conflicts of interest.
