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%.
Highlights
Synthesis and characterization of quantum dots (QD)-Cd2+(aq) + Te2-(aq) ⇌ (CdTe) functionalized with mercaptosuccinic acid (MSA)
Synthesis of QD-CdTe in aqueous media involves the formation of a complex between the metallic cation (Cd2+) in solution in the presence of the surface ligand, followed by chemical reduction of tellurium by sodium borohydride, and consequent formation of the nanoparticle
Longer synthesis times produce nanoparticles of greater size and less quantum confinement, provoking a gradual displacement to high wavelengths.[49,50]. This behavior is established for the QD-CdTe and can be confirmed in various studies.[51,52,53,54,55]
Summary
Protamine (PT) is a low molecular weight polycationic protein (from 5000 to 10,000 Da) whose composition is rich in amino acids such as arginine (> 67%), proline, serine, and valine, and which can be obtained from the sperm of salmon and other fish belonging to the family Salmonidae.[1,2] This protein can be used for deoxyribonucleic acid (DNA) purification, increase the life span of insulin in the body, and as an antibacterial agent in food.[3,4] Besides, according to Boer et al.,[5] PT is the only molecule approved by the Food and Drug Administration (FDA) to act as a neutralizing agent for heparin and is administered during procedures involving extracorporeal circulation, such as dialysis and cardiac surgery.[5,6] if misused, it can cause systemic hypotension, pulmonary hypertension, hemorrhage, sudden pressure drop, bradycardia, and dyspnea.[6,7] Due to its wide range of applications and. The bioconjugation process results from the interaction of the functionalizing agent (peptides or proteins, for example) with the analyte, which can occur from the analyte’s adsorption on the surface of the QD through non-polar and/or electrostatic interactions, coordinated binding of the QD with the analyte or vice versa, specific bonds of the antibody-antigen type and formation of covalent bonds.[42] the field of possibilities for using QD (such as CdTe), especially as photoluminescent probes, still allow exploring many systems (analytes) and applications This way, in this work, the synthesis and spectroscopic characterization of QD-CdTe, functionalized with mercaptosuccinic acid (negatively charged), was performed. The CdTe quantum dots’ synthesis was performed based on Carvalho et al.[43] with modifications In this procedure, solutions containing 100 μmol of cadmium chloride (CdCl2), 765 μmol of sodium citrate dihydrate, 100 μmol of MSA, and 200 μmol of sodium tellurite (Na2TeO3) were prepared (in a total volume of 2 mL, for each reagent), and a mass of sodium borohydride (NaBH4) equivalent to 660 μmol was weighed. In the process of optimization and validation of the method, when necessary, the results were evaluated using the Student’s t-test considering normal distribution (random error) and a 95% confidence interval
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