test tools for field measurements. Cyclic voltammetry and electrochemical impedance spectroscopy are two frequently used techniques in these applications. Both techniques measure either the current or the impedance obtained by varying the applied potential. When applied here, the method detects binding of an analyte on the surface of an PTK/ZK manufacturer electrode by the amount it hinders the access of an electroactive redox couple to October 2011 | Volume 6 | Issue 10 | e24948 Peptide-Based Biosensors the electrode, therefore decreasing the redox current in CV or increasing impedance in EIS. CV provides rapid evidence for binding but it is less quantitative. In EIS, the sinusoidal current obtained in response to small amplitude, sinusoidal perturbations of the potential is measured. The current response has an amplitude relative to the potential amplitude and is also out of phase from the perturbation. For this reason, results are reported as complex impedance. It is common to plot EIS data in a Nyquist plot, and fit them using an equivalent circuit model to extract the parameter of interest. Here it is the resistance of charge transfer of the redox couple, which is impacted by bound analyte. In this paper, we describe a novel biosensor for the detection of alanine aminotransferase. ALT catalyzes the reversible transamination of L-alanine and a-ketoglutarate to pyruvate and L-glutamate with pyridoxal 59-phosphate as a coenzyme. ALT is found mainly in the liver, but is also found in red blood cells, heart cells, muscle tissue and other organs, such as the pancreas and kidneys. Serum ALT levels are an indicator for liver damage and its detection is considered the gold standard biomarker of hepatotoxicity. The normal concentration of ALT in blood serum ranges from 535 U/L, and serum ALT levels increase up to 50-fold 
in connection with a variety of liver conditions, including viral infection, cirrhosis, non-alcoholic steatohepatitis, and drug toxicity. Most ALT biosensors are based on the detection of the enzymatic activity of the ALT enzyme as opposed to the detection of the protein itself. In this article we present the selection of ALT binding peptides from an 17062696” M13 phage displayed peptide library and the characterization of the binding affinities of the peptides. Once the most promising peptide was selected, a cysteine-modified free peptide was synthesized and transferred to the biosensor platform. The binding activity was monitored in situ by QCM, and the peptidemodified gold electrode was used to detect ALT quantitatively using EIS. The general approach can be extended to develop biosensors for a wide variety of target analytes. The methodology can be easily applied in a relatively short period of time at low cost. Biopanning of Phage-Displayed Peptide Library ALT was dissolved in NaHCO3 buffer and transferred to the wells of polystyrene microplates. After overnight incubation with mild agitation at 4uC, the well surfaces were coated with ALT. The well surfaces were then blocked with blocking buffer for 1 hr at 4uC, and washed 6 times with TBST to remove weakly bound ALT. The Ph.D.-12 21900205” phage displayed random peptide library in 100 mL of TBS buffer was added to the ALT coated wells, and the plate was shaken gently for 1 h at room temperature. Wells were washed 10 times with TBST to remove unbound phage. The bound phage were eluted with 100 mL of 0.2 M glycine-HCl and the elution was immediately neutralized with 15 mL Tris-HCl to prevent phage killing. The