**4. Concluding remarks**

**Table 1** summarizes the reported interaction energies of boron-based clusters with the bio-molecules considered. Interaction energy constitutes the main parameter to evaluate the suitability of a carrier molecule for biomedical applications. Although the number of studies is rather limited, an analysis of these theoretical results suggests that boron-based clusters deserve to be regarded as promising candidates for the bio-sensing and drug delivery-related applications.

A large interaction energy indicates a stable complex, but it invariably causes a longer recovery time, which is not a good factor for drug release or for refreshing a biosensor. Thus, a semi-chemisorption with an effective recovery time (less than 1 second) is more favorable for biomedical applications. Based on Eq. (2), the recovery of a biomedical agent from a typical carrier surface is estimated to be short in the range 0.03–0.06 microsecond for NIR light with the typical interaction energy of −10 kcal/ mol at 310 K of the human body. Also, it amounts to 0.3–0.7 second for interaction energies of −20 kcal/mol, which seems to be usable for an appropriate biosensor [107].

The recovery time exponentially increases by an enhancement of interaction energy. According to **Table 1**, most of the reported interaction energies are smaller than −20 kcal/mol, with an expected error margin of ±3 kcal / *mol* for DFT computations, which provide suitable recovery times at human body temperature in the range of NIR light.

In spite of the frequent claims of many authors in reported theoretical studies, some systems are not suitable due to a long recovery time (in the order of second or even longer). For example, in the aforementioned condition, B36 suffers from long recovery time for sensing cytosine and guanine; similarly, C4B32 suffers from long recovery time for detecting 6-thioguanine, nitrosourea, and cisplatin drugs.

Noticeably, the release mechanism of drug is also a crucial factor, which should be understood for a better design of a drug delivery system, in addition to suitable recovery time. Such studies are yet to be carried out.

In summary, from a theoretical viewpoint, boron-based clusters having some unique structural and electronic properties provide us with great potential in biomedical applications. Quantum chemical calculations can further assist experimental researchers in the understanding of these systems from the molecular insights at low cost but with much detail and substantial accuracy.
