## 1 Introduction
### Modified Electrode Types
#### 1.1.1 Chemically Modified Electrodes [27; 33]  
Chemically modified electrodes are electrodes that have been modified with chemicals that can bind to specific species in solution [34]. These chemicals are often referred to as \"functional groups,\" and they can be tailored to bind to a particular species of interest. In particular, chemically modified electrodes are conducting or semiconducting materials that have been covered with a film made of a single molecule, multiple molecules, ions, or polymers. This film changes the electrochemical, optical, and other characteristics of the interface. The conductive and semiconductive substrates are typically made from standard electrode materials, while the adlayers can come from a variety of sources and have diverse properties. This diversity expands the range and capabilities of electrochemical techniques [35]. Molecules that are electrochemically reactive are the most useful for attaching to functionalized electrodes. For instance, ruthenium complexes with carboxylic acid groups can be coupled to an alkylamine-silanized Pt oxide surface. By applying a potential to a Pt electrode, the immobilized Ru(II) complex can be made to undergo the same oxidation and reduction reactions as it would in solutions with electron acceptors or donor reagents. In cyclic voltammetry, this results in an anodic current peak at +0.79 V when the potential is swept linearly. The Ru(III) complex product remains attached to the Pt and can be reduced in a reverse potential sweep, resulting in a cathodic current peak [36].  
Chemically modified electrodes are often used in electroanalytical chemistry, where they are used to measure the concentration of various species in solution. They can also be used in sensors and other types of analytical instruments. One advantage of chemicallymodified electrodes is that they can be highly selective for a particular species, which makes them useful for detecting trace amounts of a specific analyte. However, they can also be prone to fouling and instability, which can limit their performance [34].  
Any conventional electrode material can be used as a substrate, but some materials are easier to modify. The most commonly modified conductive metals are Au, Ag, and Pt, as these surfaces can be easily reproduced and kept clean in the laboratory. Carbon electrodes are also frequently used for similar reasons. Other metals and semiconductors are used less often but are still important materials [34].  
In antibiotics detection, different electrochemical sensing strategies were achieved by the combination of bio-recognition molecules (antibodies, enzymes, and aptamers) with the unique sensitivity of metal nanoparticles (NPs) [27]. Therefore, daunomycin was selectively and sensitively detected using a label-free electrochemical aptasensing method [37]. Here, Au NPs were functionalized with 2, 2'.5'2''-terthiophene-3' (p-benzoic acid) used for the effective co-immobilization of aptamers and phosphatidylserine. The authors tested the effect of different factors such as reaction time, temperature, immobilized aptamer concentration, and pH to obtain the maximal sensitivity; a detection limit of 0.053 nM over a concentration range of 0.1 to 61 nM toward the antibiotic daunomycin was found. The fabricated sensor provided brilliant selectivity, sensitivity, and stability toward daunomycin detection in real samples of human urine. Similarly, kanamycin was detected using another fabricated label-free electrochemical aptasensor constructed by covalent immobilization of a kanamycin-specific DNA aptamer over Au NPs previously modified with poly-[2,5-di(2-thienyl)-1H-pyrrole-1-(p-benzoic acid)] [38]. Using LSV (linear sweep voltammetry) as a transduction method, this fabricated aptasensor provided a detection limit of 9 nM over a concentration range of 0.06 to 10 \(\upmu\)M.