Topics in Current Chemistry (2020) 378:35
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The main elements of the bioconjugation process requiring attention have been
described in a comprehensive article written by Sapsford et al. [56] and comprise:
(1) Control over the BM/QD ratio. The desirable ratio varies with the type of the
application. It should be noted that QDs are usually larger than BMs, with the
possible exception of some large proteins. It should also be noted that the reactions are interfacial in nature and that such interfaces inherently polydisperse
across a population of QDs
(2) Control over the orientation of the BM on the QD. Optimal activity of both the
QD and the BM should be maintained.
(3) Control over the separation between QD and BM. This point is crucial if the
platform QD–BM is to be used in a Förster resonance energy transfer (FRET)
experiment [59]
(4) Control over the strength of the QD–BM bond. Most clinical or in vivo experiments require permanent and, therefore, strong linkage.
Many different approaches can be used to immobilize BMs onto the QD surface,
with the simplest method to link a BM to a QD surface being adsorption. However, in adsorption, the attachment of a BM is rather tenuous and maintained by
weak interactions only, such as hydrogen bonding, London dispersion and Coulombic forces and lone-pair electrons [60]. As an example, the proteins present in the
human body tend to bind nonspecifically onto the surface of QDs, a process which is
to be avoided. A closely related approach is based on pure electrostatic interactions
between the BM and the QD [61]. However, while this method of functionalization is generally straightforward and fast, electrostatic interaction, similar to simple
adsorption, suffers from serious disadvantages, including instability, lack of orientation control on the BM and on the BM/QD ratio [62, 63]. Therefore, the most common routes to achieve bioconjugation consist of the four shown in Fig. 6 [56].
4.1 Direct Union of BMs
BMs can be joined to a semiconductor QD surface through the direct covalent union
of the BMs to the surface of the QD semiconductor (route 1 in Fig. 6). Thus, proteins, peptides and nucleic acids can be bonded to the QD surface metal atoms (especially Zn) through their (cysteine) thiol and (histidine) imidazole groups [56]. Thiol
and histidine motifs could eventually be added to the “natural” BMs and, occasionally, thiol groups are created, reducing peripheral S–S bonds. Occasionally, BMs are
bonded directly to the QD in the synthesis processes (biological templating) [56].
4.2 BMs Bonded to a Ligand
A more general method to join BMs to a QD surface is to bind covalently the BM
to a ligand that has been previously attached to the QD surface (route 2 in Fig. 6). It
should be pointed out that GQDs usually contain carboxyl, hydroxyl, carbonyl and
epoxide external groups [64] and that carbon-based QDs (CQDs) can be synthesized
144
Reprinted from the journal
1 3
The main elements of the bioconjugation process requiring attention have been
described in a comprehensive article written by Sapsford et al. [56] and comprise:
(1) Control over the BM/QD ratio. The desirable ratio varies with the type of the
application. It should be noted that QDs are usually larger than BMs, with the
possible exception of some large proteins. It should also be noted that the reactions are interfacial in nature and that such interfaces inherently polydisperse
across a population of QDs
(2) Control over the orientation of the BM on the QD. Optimal activity of both the
QD and the BM should be maintained.
(3) Control over the separation between QD and BM. This point is crucial if the
platform QD–BM is to be used in a Förster resonance energy transfer (FRET)
experiment [59]
(4) Control over the strength of the QD–BM bond. Most clinical or in vivo experiments require permanent and, therefore, strong linkage.
Many different approaches can be used to immobilize BMs onto the QD surface,
with the simplest method to link a BM to a QD surface being adsorption. However, in adsorption, the attachment of a BM is rather tenuous and maintained by
weak interactions only, such as hydrogen bonding, London dispersion and Coulombic forces and lone-pair electrons [60]. As an example, the proteins present in the
human body tend to bind nonspecifically onto the surface of QDs, a process which is
to be avoided. A closely related approach is based on pure electrostatic interactions
between the BM and the QD [61]. However, while this method of functionalization is generally straightforward and fast, electrostatic interaction, similar to simple
adsorption, suffers from serious disadvantages, including instability, lack of orientation control on the BM and on the BM/QD ratio [62, 63]. Therefore, the most common routes to achieve bioconjugation consist of the four shown in Fig. 6 [56].
4.1 Direct Union of BMs
BMs can be joined to a semiconductor QD surface through the direct covalent union
of the BMs to the surface of the QD semiconductor (route 1 in Fig. 6). Thus, proteins, peptides and nucleic acids can be bonded to the QD surface metal atoms (especially Zn) through their (cysteine) thiol and (histidine) imidazole groups [56]. Thiol
and histidine motifs could eventually be added to the “natural” BMs and, occasionally, thiol groups are created, reducing peripheral S–S bonds. Occasionally, BMs are
bonded directly to the QD in the synthesis processes (biological templating) [56].
4.2 BMs Bonded to a Ligand
A more general method to join BMs to a QD surface is to bind covalently the BM
to a ligand that has been previously attached to the QD surface (route 2 in Fig. 6). It
should be pointed out that GQDs usually contain carboxyl, hydroxyl, carbonyl and
epoxide external groups [64] and that carbon-based QDs (CQDs) can be synthesized
144
Reprinted from the journal