Overview of the main methods used to combine proteins with nanosystems: absorption, bioconjugation, and encapsulation

Figures & data

Table 1 The most popular cross-linker reagents for coupling protein to nanoparticle based on their respective functions

Reactive groups	Eg of functional cross-linker	Functional groups on nanoparticles/proteins	Functional groups on proteins/nanoparticles
–NHS ester Maleimide or Iodoacetamides	SIAB, SMCC, SPDP, SPMB, MBS	–NH2	–SH
Carbodiimide	EDC or EDAC + sulfo-NHS stabilizer	–COOH	–NH2
–NHS ester	EGS, DSP, DSS, BS3	–NH2	–NH2
Maleimide	BMME	–SH	–SH

Abbreviations: SIAB, N-succinimidyl(4-iodoacetyl)aminobenzoate; SMCC, succinimidyl-4-(N-maleimidomethyl)cyclohexana-1-carboxylate; MBS, m-maleimidobenzoyl-N-hydroxysuccimide ester; SPDP, succinimidyl 3-(2-pyridyldithio)propionate; SPMB, succinimidyl (4-p-maleimidophenyl)butyrate; EDC, 1-Ethyl-3-[3-dimethylaminopropyl] carbodiimide hydrochloride; EGS, ethylene glycolbis(succinimidylsuccinate); BS3, bis-(sulfosuccinimidyl) suberate; BMME, bis(maleimido methyl) ether; DSS, disuccinimidyl suberate; DSP, dithiobis (succinimidyl propionate).

Figure 1a The introduction of sulfhydryl groups by: 1) the reduction of protein disulfide bonds using reductive agents such as dithiotreitol (DTT = Cleland’s reagent). 2) Coupling protein primary amino groups with 2-iminothiolane (Traut; s reagent).

Figure 1b The introduction of sulfhydryl groups by: 3) Quenching of reactive protein aldehyde residues with cystaminiumdichloride reagents or 4) coupling of cystaminiumdichloride to carboxyl groups via 1-ethyl-3-(3-dimethyl-aminopropyl)carbodiimide (EDC); both cases followed by the disulfide bonds reduction with DTT.

Figure 2 Protein–nanoparticle interactions: main factors that can affect proteins resulting in their denaturation. In this example, proteins are conjugated on amino functionalized nanoparticles using the cross-linker SPMB.

Figure 3 Entrapment of enzymes using sol-gel chemistry: A schematic overview of the sol-gel process. Several silicate precursors can be used to modify the surface chemistry of the sol-gels such as TMSO, APTES, MTMOS, and ETMOS.

Abbreviations: TMSO, tetramethyl orthosilicate; APTES, 3-aminopropyltriethoxysilane; MTMOS, methyltrimethoxysilane; ETMOS, ethyltrimethoxysilane.