Electrochemical detection of nanomolar dopamine in the presence of neurophysiological concentration of ascorbic acid and uric acid using charge-coated carbon nanotubes via facile and green preparation

• Charge-doped MWCNTs were prepared by simple sonication with CA or PEI and MWCNT.
• DA was clearly determined at the nM level with ~105 times concentrated AA.
• LOD of CA/MWCNT/GC is 4.2 nM, which is 5.2 times better than that of MWCNT/GC.
• This DA sensor is useful even under neurophysiological concentration of AA and UA.
• Our sensor is applicable to early diagnosis and moreover simple and eco-friendly.

Negatively charged multi-walled carbon nanotubes (MWCNTs) were prepared using simple sonication technique with non-toxic citric acid (CA) for the electrochemical detection of dopamine (DA). CA/MWCNTs were placed on glassy carbon (GC) electrodes by drop-casting method and then electrochemical determinations of DA were performed in the presence of highly concentrated ascorbic acid (AA). For the comparison of the charge effect on MWCNTs surface, positively charged polyethyleneimine (PEI)/MWCNT/GC electrode and pristine MWCNT/GC electrode were also prepared. Contrary to conventional GC electrode, all three types of MWCNT modified electrodes (CA/MWCNT/GC, PEI/MWCNT/GC, and pristine MWCNT/GC) can discriminate ~μM of DA from 1 mM AA using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) due to the inherent electrocatalytic effect of MWCNTs. Compared to positively charged PEI/MWCNT/GC and pristine MWCNT/GC electrodes, negatively charged CA/MWCNT/GC electrode remarkably enhanced the electrochemical sensitivity and selectivity of DA, showing the linear relationship between DPV signal and DA concentration in the range of 10–1000 nM even in the presence of ~105 times concentrated AA, which is attributed to the synergistic effect of the electrostatic interaction between cationic DA molecules and negatively charged MWCNTs and the inherent electrocatalytic property of MWCNT. As a result, the limit of detection (LOD) of DA for CA/MWCNT/GC electrode was 4.2 nM, which is 5.2 and 16.5 times better than those for MWCNT/GC electrode and PEI/MWCNT/GC electrode even in the presence of 1 mM AA. This LOD value for DA at CA/MWCNT/GC electrode is one of the lowest values compared to the previous reports and is low enough for the early diagnosis of neurological disorder in the presence of physiological AA concentration (~0.5 mM). In addition, the high selectivity and sensitivity of DA at CA/MWCNT/GC electrode were well kept even in the presence of both 1 mM AA and 10 μM uric acid (UA) as similar as neurophysiological concentration.

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