Bioconjugation Between CdTe Quantum Dots and a Cationic Protein: An Analytical Method to Determine Protamine in Drug and Urine Samples

Abstract

CdTe quantum dots (QD-CdTe) functionalized with mercaptosuccinic acid (MSA) were synthesized in an aqueous medium, varying synthesis time from 0.5 to 4 h. The nanoprobe were characterized by a direct relationship between synthesis time and QD size (2.61-3.04 nm). The QD-CdTe-MSA interacted with protamine (PT), a cationic protein, forming a bioconjugate, thus quenching the photoluminescence intensity and generating an on-off system. The nanoprobe produced at a synthesis time of 1 h (QD-CdTe1) presented PT’s best sensitivity in a succinate buffer (pH = 5). Under the optimized conditions, the proposed method presented a linear range of 0.05-0.5 mg L-1 (10-100 nM), limit of detection (LOD) 0.01 mg L-1 (2 nM), and relative standard deviation (RSD) ≤ 2.01% (n = 10). The interaction of the nanoprobe and PT led to aggregation due to a bioconjugate formation. The systems’ hydrodynamic radius varied from 4.31 nm (QD‑CdTe1) to 30.50 nm for the bioconjugate (QD-CdTe1-PT). The method was sensitive to variation in ionic strength and based on thermodynamic parameters; it was demonstrated that the interaction mechanism occurred preferentially through electrostatic forces. Finally, the method proved to be fast, sensitive, and viable for quantifying PT in drugs and synthetic urine samples with recoveries above 95%.

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“…The QDs were functionalized with mercaptosuccinic acid (MSA), according to the following procedure: a mixture of cadmium chloride (CdCl 2 ) at 100 μmol, sodium citrate dihydrate at 765 μmol, MSA at 100 μmol and sodium tellurite (Na 2 TeO 3 ) was prepared; then sodium borohydride (NaBH 4 ) (660 μmol) was prepared; 17 ml of ultrapure water was added in a round-bottom flask, afterwhich, the components were added in a precise order; the mixture was heated at 90°C, under reflux; the CdTe-MSA modified QDs were precipitated with ethanol and separated by centrifugation (washed 3 times, 15 min at 3000 RPM). The modified particles were resuspended in 10 ml of water and stored away from light, at 4°C ( da Costa et al, 2021 ).…”

Mannose-binding lectin conjugated to quantum dots as fluorescent nanotools for carbohydrate tracing

“…The stabilizing/functionalizing agent, MSA, confers carboxylic groups and negative charges to the QD surface, allowing to conjugate the nanocrystals with biomolecules [41]. Thus, two approaches were evaluated to conjugate the MSA-coated QDs with rhMBL: (i) adsorption interaction between the nanocrystals and rhMBL; and (ii) a covalent approach, using coupling agents to provide chemical bonds between carboxyl moieties presented on the QD surface and amine terminals of rhMBL.…”