Covalently grafting first-generation PAMAM dendrimers onto MXenes with self-adsorbed AuNPs for use as a functional nanoplatform for highly sensitive electrochemical biosensing of cTnT

Abstract

2D MXene-Ti₃C₂T_χ has demonstrated promising application prospects in various fields; however, it fails to function properly in biosensor setups due to restacking and anodic oxidation problems. To expand beyond these existing limitations, an effective strategy to for modifying the MXene by covalently grafting first-generation poly(amidoamine) dendrimers onto an MXene in situ (MXene@PAMAM) was reported herein. When used as a conjugated template, the MXene not only preserved the high conductivity but also conferred a specific 2D architecture and large specific surface areas for anchoring PAMAM. The PAMAM, an efficient spacer and stabilizer, simultaneously suppressed the substantial restacking and oxidation of the MXene, which endowed this hybrid with improved electrochemical performance compared to that of the bare MXene in terms of favorable conductivity and stability under anodic potential. Moreover, the massive amino terminals of PAMAM offer abundant active sites for adsorbing Au nanoparticles (AuNPs). The resulting 3D hierarchical nanoarchitecture, AuNPs/MXene@PAMAM, had advanced structural merits that led to its superior electrochemical performance in biosensing. As a proof of concept, this MXene@PAMAM-based nanobiosensing platform was applied to develop an immunosensor for detecting human cardiac troponin T (cTnT). A fast, sensitive, and highly selective response toward the target in the presence of a [Fe(CN)₆]^3-/4- redox marker was realized, ensuring a wide detection of 0.1-1000 ng/mL with an LOD of 0.069 ng/mL. The sensor's signal only decreased by 4.38% after 3 weeks, demonstrating that it exhibited satisfactory stability and better results than previously reported MXene-based biosensors. This work has potential applicability in the bioanalysis of cTnT and other biomarkers and paves a new path for fabricating high-performance MXenes for biomedical applications and electrochemical engineering.

Keywords: Biosensors; Electronic properties and materials.

Fig. 1. Schematic illustration of the fabrication of the AuNPs/MXene@PAMAM platform and its electrochemical application for cTnT detection.

a 2D Ti₃C₂T_χ nanosheets-MXene prepared by acid etching aluminum layers from Ti₃AlC₂, were covalently grafted with the first-generation PAMAM dendrimers by in-situ growth method. The MXene@PAMAM supernatant obtained by sonication and centrifugation was introduced onto the surface of disposable SPCE by drop-casting method. Then, the massive amino terminals of PAMAM offered abundant active sites for adsorbing AuNPs to construct 3D heterogeneous nanoarchitecture. Subsequently, thiol-linked antibodies against cTnT were introduced into AuNPs/MXene@PAMAM as the recognition element in the electrochemical immunosensor. The developed cTnT immunosensor were capable of monitoring increased serum cTnT that were valuable indicator of cardiomyocyte damage. SEM images of b Ti₃AlC₂-MAX, c Ti₃C₂T_χ-MXene, and d MXene@PAMAM.

Fig. 2. Schematic of the procedure for synthesizing the MXene@PAMAM (abbreviated from MXene@G1.0PAMAM).

Ti₃C₂T_χ-MXene featured abundant hydroxyl (-OH) groups that were introduced during HF etching process. These highly active -OH groups easily underwent a ring-opening esterification reaction with succinic anhydride in an anhydrous environment, through which the carboxylic acid functional groups were covalently introduced into the surface of the Ti layer to obtain carboxyl-MXene and for further growth of PAMAM. The as-prepared carboxylates were then reacted with SOCl₂ to perform acyl chloride reaction, followed by treating with ethylenediamine to obtain the dendrimer initiator MXene@G_0.0PAMAM. The initiator was then served as substrate and core to accomplish the subsequent reaction of interior branch cells and terminal branch cells. After one cycle grafting reaction, MXene was modified with the first-generation PAMAM dendrimer (MXene@PAMAM).

Fig. 3. Structural characterization of MXene@PAMAM.

a XRD patterns of Ti₃AlC₂-MAX, Ti₃C₂T_χ-MXene, and MXene@PAMAM. b FTIR spectra of Ti₃C₂T_χ-MXene, carboxylic MXene (MXene-COOH), and MXene@PAMAM. c Powder XPS survey spectrum of Ti₃C₂T_χ-MXene and MXene@PAMAM. High-resolution XPS spectrum of d C 1s spectrum of Ti₃C₂T_χ-MXene, and e C 1s and (f) N 1s spectra of MXene@PAMAM.

Fig. 4. Morphologic characterization of AuNPs/MXene@PAMAM.

TEM images of a the MXene, and b, c few-layer MXene@PAMAM flakes. d UV–Vis spectrum of AuNPs (inset: photograph of AuNP suspension). e Average size and size distribution of AuNPs (inset: TEM images of AuNPs). f High-resolution TEM (HRTEM) of AuNPs (inset: SEAD of AuNPs). g TEM images of AuNPs/MXene@PAMAM (inset: the corresponding SEAD) and h the corresponding TEM-EDS elemental map. The particle size of the AuNPs was measured with the ImageJ software.

Fig. 5. Biosensing properties of AuNPs/MXene@PAMAM nanohybrid.

a CVs of the MXene, MXene@PAMAM, AuNP, and AuNP/MXene@PAMAM modification on the SPCE. b MXene@PAMAM/SPCE with 50 CV cycles and c the corresponding redox peaks. d Nyquist plots of the bare SPCE, NaOH activation/SPCE, MXene@PAMAM/SPCE, and AuNPs/MXene@PAMAM/SPCE recordings from 0.1 to 100,000 Hz (inset: Randles equivalent circuit of the electrochemical impedance data; Rs: solution impedance, Rct: charge-transfer impedance, Z_w: Warburg impedance, C_dl: double-layer capacitance of the electrode/electrolyte interface). e Amplification effect after modification on DPV signal. The modified electrodes were the same as in d. f Randles–Sevcik plots, the oxidation peak current of the bare SPCE, NaOH activation/SPCE, MXene@PAMAM/SPCE, and AuNPs/MXene@PAMAM/SPCE versus the square roots of the scan rate from 30 to 100 mV/s. All the above experiments were performed in 0.01 M PBS containing 5 mM [Fe(CN)₆]^3−/4− and 0.1 M KCl.

Fig. 6. CV characterization and analytical performance of the immunosensor.

a CVs of a bare SPCE, b NaOH-activated SPCE, c MXene@PAMAM/SPCE, d AuNPs/MXene@PAMAM/SPCE, e SH-mAb/AuNPs/MXene@PAMAM/SPCE, f BSA/SH-mAb/AuNPs/MXene@PAMAM/SPCE, and g cTnT/BSA/SH-mAb/AuNPs/MXene@PAMAM/SPCE. b Sensitivity comparison of anti-cTnT mAb- and thiol-linked mAb-based immunosensors using DPV measurement of cTnT antigen (curve from top to bottom: 1, 10, and 100 ng/mL). c DPV response of the proposed immunosensor toward different concentrations of cTnT (curve from top to bottom: 0.1, 1, 10, 50, 100, 500, 1000 ng/mL); the peak voltage was ~0.15 V. d Calibration curve of the DPV peak currents for various concentrations of cTnT and obtained an LOD of 0.069 ng/mL. e Specificity assay of the proposed cTnT immunosensor. f Stability study of the proposed cTnT immunosensor. Error bars represented standard deviation (n = 3).