Electrocatalytic oxidation of D-penicillamine (D-PA) at the surface of ferrocene modified carbon paste electrode (FCCPE) was thoroughly investigated in aqueous solution with various pH. The performance of this modified electrode was compared with those of 2,7-bis(ferrocenyl ethyl) fluoren-9-one modified carbon paste electrode (2,7-BFEFMCPE). In the optimum condition, the oxidation of D-PA at the surface of FCCPE and 2,7-BFEFMCPE is occurred about 480 and 320 mV less positive than that unmodified carbon paste electrode, respectively. The linear dynamic ranges $6 \times 10^{-5}$ M$^{-2} \times 10^{-3}$ M, $6.5 \times 10^{-5}$ M-$1.1 \times 10^{-3}$ M and $7\times 10^{-6}$ M-$1.6 \times 10^{-4}$M, $7 \times 10^{-6}$ M-$2 \times 10^{-4}$ M of D-PA are obtained from CV and DPV methods for FCCPE and 2,7-BFEFMCPE, respectively. The detection limits (3𝜎) were determined as $5.4 \times 10^{-5}$ M and $6.3 \times 10^{-5}$ M in CV and $6.2 \times 10^{-6}$ M and $6.8 \times 10^{-6}$ M in DPV determinations for FCCPE and 2,7-BFEFMCPE, respectively. The proposed method was applied in a highly sensitive determination of D-PA in drug and human synthetic serum samples by standard addition and recovery methods, respectively.

The preparation and electrochemical performance of the carbon nanotube paste electrode modified with ferrocene (FCMCNPE) was investigated for electrocatalytic behaviour toward oxidation of 𝑁-acetyl-𝐿-cysteine (NAC) in the presence of tryptophan (Trp) using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The results showed an efficient electrocatalytic activity of FCMCNPE toward oxidation of NAC and Trp, as the electrooxidation of NAC and Trp together gave two well-defined anodic peaks, revealing the applicability of this modified electrode for simultaneous voltammetric detection of mentioned compounds in the same solution. The values of catalytic rate constant (𝑘) and the apparent diffusion coefficient (D_app) were also calculated using chronoamperometry. The DPV method was applied as a sensitive method for the quantitative detection of trace amounts of NAC and Trp. A linear dynamic range from 1.0 to 18.0 𝜇M for NAC and 2.0 to 150.0 𝜇M for Trp was obtained using DPV method in pH 7.00 buffered solution and the detection limit (3𝜎) was determined as 0.49 𝜇M and 0.54 𝜇M for NAC and Trp, respectively. The proposed method was also applied for analysis of NAC tablet, investigating the applicability of the proposed voltammetric method for determination of NAC in real sample.