Optical Detection of Ketoprofen by Its Electropolymerization on an Indium Tin Oxide-Coated Optical Fiber Probe

Abstract

In this work an application of optical fiber sensors for real-time optical monitoring of electrochemical deposition of ketoprofen during its anodic oxidation is discussed. The sensors were fabricated by reactive magnetron sputtering of indium tin oxide (ITO) on a 2.5 cm-long core of polymer-clad silica fibers. ITO tuned in optical properties and thickness allows for achieving a lossy-mode resonance (LMR) phenomenon and it can be simultaneously applied as an electrode in an electrochemical setup. The ITO-LMR electrode allows for optical monitoring of changes occurring at the electrode during electrochemical processing. The studies have shown that the ITO-LMR sensor’s spectral response strongly depends on electrochemical modification of its surface by ketoprofen. The effect can be applied for real-time detection of ketoprofen. The obtained sensitivities reached over 1400 nm/M (nm·mg^−1·L) and 16,400 a.u./M (a.u.·mg^−1·L) for resonance wavelength and transmission shifts, respectively. The proposed method is a valuable alternative for the analysis of ketoprofen within the concentration range of 0.25⁻250 μg mL^−1, and allows for its determination at therapeutic and toxic levels. The proposed novel sensing approach provides a promising strategy for both optical and electrochemical detection of electrochemical modifications of ITO or its surface by various compounds.

Keywords: anti-inflammatory drug; drug analysis; electrochemistry; electropolymerization; indium tin oxide (ITO); ketoprofen; lossy-mode resonance (LMR); optical fiber sensor; reactive magnetron sputtering thin film.

Figure 1

The schematic representation of the experimental setup with ITO-LMR probe used for combined optical and electrochemical KP detection. The electrodes were denoted as working (WE), reference (RE), and counter (CE).

Figure 2

Spectral response of ITO-LMR probe to changes in external RI (n). The changes of resonance wavelength (λ_R) and transmission (T) at λ = 600 nm are shown in the inset.

Scheme 1

Chemical structure of KP and mechanism of its electrochemical reduction.

Figure 3

Cyclic voltammetry curves recorded for (a) GC electrode in 0.1 M phosphate buffer saline containing 2 mM of KP for 10 cycles, scan rate of 50 mV·s⁻¹; and (b) bare GC and GC/KP electrode in 0.5 M Na₂SO₄ containing of 5 mM [Fe(CN)₆]^3−/4−. The scan rate was set to 100 mV·s⁻¹.

Figure 4

Cyclic voltammetry curves recorded for (a) ITO electrode in 0.1 M phosphate buffer saline containing 2 mM of KP (10 cycles, scan rate of 50 mV·s⁻¹) and (b) ITO and ITO/KP electrode in 0.5 M Na₂SO₄ containing 5 mM [Fe(CN)₆]^3−/4−, scan rate 100 mV·s⁻¹.

Figure 5

Cyclic voltammetry curves recorded for (a) ITO-LMR electrode in 0.1 M phosphate buffer saline containing 2 mM of KP for 6 cycles at scan rate of 50 mV·s⁻¹; and (b) bare for and KP-modified ITO-LMR in 0.5 M Na₂SO₄ containing 5 mM [Fe(CN)₆]^3−/4−, scan rate 100 mV·s⁻¹.

Figure 6

XPS survey spectrum and high-resolution XPS spectra registered for C1s and O1s energy range. Peaks underwent spectral deconvolution are superimposed with colors depending on their origination (blue for KP and green for ITO). The KP concentration was 1 × 10⁻³ M.

Figure 7

Changes in optical response of the ITO-LMR probe recorded during electropolymerization of KP on ITO surface for two KP concentrations, namely (A) 1 × 10⁻⁶ M and (B) 1 × 10⁻³ M.

Figure 8

Change in resonance wavelengths (λ_R) {×} and transmission (T) at 600 nm {□} with progress of KP electropolymerization process on ITO-LMR probe for KP concentration (A) 1 × 10⁻⁶ M; (B) 1 × 10⁻⁵ M; (C) 1 × 10⁻⁴ M; and (D) 1 × 10⁻³ M.