Psychiatric disorders are common and complex diseases that show polygenic and multifactorial heredity. A single nucleotide polymorphism (Val108/158Met) in the catechol-O-methyl transferase (COMT) gene is related to many psychiatric disorders such as schizophrenia, alcoholism, bipolar disorder, and obsessive-compulsive disorder. Schizophrenia. is a complex disorder and a single nucleotide polymorphism (Val108/158Met) at the COMT gene is related to schizophrenia susceptibility. A novel hybridization-based disposable electrochemical DNA biosensor for the detection of a common functional polymorphism in the COMT gene from polymerase chain reaction (PCR) amplicons has been described without using an external label. This developed technology combined with a disposable carbon graphite electrode and differential pulse voltammetry was performed by using short synthetic oligonucleotides and PCR amplicons in length 203 bp to measure the change of guanine oxidation signal obtained at similar to+1.0 V after DNA hybridization between probe and target (synthetic target or denatured PCR samples). COMT-specific oligonucleotides were immobilized onto the carbon surface with a simple adsorption method in two different modes: (a) Guanine-containing targets were attached or (b) inosine-substituted probes were attached onto an electrode. By controlling the surface coverage of the target DNA, the hybridization event between the probes and their synthetic targets or specific PCR products was optimized.The wild-type or polymorphic allele-specific probes/targets were also interacted with an equal amount of noncomplementary and one-base mismatch-containing DNAs in order to measure the sensor selectivity. The decrease or appearance in the intrinsic guanine signal simplified the detection procedure and shortened the assay time because protocol eliminates the label-binding step. The nonspecific binding effects were minimized by using sodium dodecyl sulfate with different washing methods. The Val108/158Met COMT genotype detection were performed with real samples containing wild-type (healthy controls), polymorphic (mutant type), and heterozygous PCR products. The detection limit (S/N = 3) of the biosensor was 2.44 pmol of target sequence in the 30-mu L samples. Analytical performance of the sensor is described, along with future prospects.