Fentanyl is a potent synthetic opioid drug used as an analgesic and anesthetic. It is approximately 100 times more potent than morphine and 50 times more potent than heroin. However, illicit fentanyl is distributed and sold illegally around the world on the black market. A fentanyl overdose may result in stupor, changes in pupil size, cold and clammy skin, cyanosis, coma, and respiratory failure leading to death. Two milligrams of fentanyl can be lethal depending on body size, tolerance, and past usage.

Identification and detection are imperative because fentanyl-related overdoses have rapidly become a major public health crisis in many communities in countries such as the USA and Canada.

The development of new methods based on the combination of electrochemical surface-enhanced Raman spectroscopy (EC-SERS) and screen-printed electrodes (SPEs) provides a rapid, efficient, and accurate approach for the detection of fentanyl [1].

Detection of fentanyl (Figure 2) was performed by the electrochemical activation of metallic SPEs concurrently with the drug’s presence in solution. The protocol consists of two steps in a single experiment: (1) the electrochemical generation of metallic nanostructures with SERS properties and (2) detection of fentanyl present in the solution.

Two SPEs were evaluated—gold (220BT) and silver (C013)—due to the enhancement of Raman intensity associated with these electrodes.

Detection of fentanyl with 220BT was performed in 1 × 10^-5 mol/L fentanyl and 0.1 mol/L KCl by cyclic voltammetry, scanning the potential from +0.70 V to +1.40 V and back to -0.20 V, with a scan rate of 0.05 V/s (Figure 3a).

Experiments with C013 were carried out in 1 × 10^-5 mol/L fentanyl, 0.1 mol/L HClO₄, and 0.01 mol/L KCl. The potential was scanned from 0.00 V to +0.40 V and back to -0.40 V, with a scan rate of 0.05 V/s (Figure 3b).

Spectroelectrochemical detection with both SPEs is based on the same methodology: the initial oxidation of the metallic surface followed by its reduction to generate Au or Ag nanoparticles (NPs) with a SERS effect. Although the characteristic Raman bands of fentanyl are detected once these nanostructures are generated, the highest Raman intensity was obtained during the final part of the experiment (+0.50 V, anodic scan) with 220BT, and at -0.40 V when working with C013.

Figure 4 displays the characteristic spectrum of fentanyl obtained with Au and Ag SPEs. Different bands are detected, with the most intense and representative band located at 1000 cm^-1.

Table 2 summarizes the assignment of the observed Raman bands with the characteristic vibrational modes of fentanyl. The interaction of fentanyl with Au and Ag SERS substrates is not identical; some vibrational modes are only detected with one metal, and the shifting of several bands is also observed.

In order to demonstrate the usefulness of this method, the intensity of Raman band at 1000 cm^-1 obtained with 220BT was analyzed with varying fentanyl concentrations. The calibration curve in Figure 5 shows linear behavior of the Raman intensity from 1 × 10^-6 mol/L (0.33 μg/mL) to 1 × 10^-5 mol/L (3.37 μg/mL) fentanyl. The high correlation coefficient value (R²  = 0.997) ensures the suitability and the sensitivity of this EC-SERS method for the detection of fentanyl in the mentioned concentration range.

The development of a sensitive fentanyl detection method based on the SERS effect is achieved. Au and Ag SPEs provide interesting results which are not only useful in the characterization of fentanyl, but also for other analytical purposes. The electrochemical activation of 220BT and C013 SPEs along with the detection of fentanyl in a single experiment represents a rapid and easy procedure that facilitates the spectroelectrochemical measurements. The calibration curve obtained with 220BT exhibits linear behavior from 1 × 10^-6 mol/L (0.33 μg/mL) to 1 × 10^-5 mol/L (3.37 μg/mL) fentanyl, demonstrating the wide potential of this method.