# Current Status of HbA1c Biosensors
## 3 Analysis of Samples from Human Blood
Even though gold has been commonly used in the biosensor system, carbon or membrane-based materials have been adopted into for their biocompatibility and cost effective feature. [PERSON] constructed an 3-APBA-modified ESMs electrode to detect HbA1c and a Hp-modified ESMs electrode to determine the Hb [32]. These sensors can simultaneously detect total Hb and HbA1c with excellent precision and also an acceptable reproducibility of fabrication. The chemical stability of the Hp-modified ESMs is good (98.84% over a shelf-life of 4 weeks), however, for the APBA-modified ESMs, the stability is not so good (92.35% over a one-week period). Thus one needs to improve the stability of the APBA-modified ESM for clinical application. In addition to the electrochemical signal transduction pathway, optical detection methods are also used in the HbA1c biosensor systems. [PERSON] and coworkers fabricated a HbA1c-capturing interface made of carboxy-EG6-undecanethiol SAM coupled with 3-APBA on a gold thin-film substrate, and used an 11-amino-1-undecanethiol SAM modified gold substrate for Hb immobilization [73]. The chemical luminescence (CL) response in the Luminol/H\({}_{2}\)O\({}_{2}\) system in which the four heme groups play a role as the catalyst is linearly proportional to the amount of Hbs. The luminol CL method not only provides a high sensitivity for HbA1c detection without the need for signal amplification, but also can be used for Hb detection on an amino-SAM based interface, which means this method can be applied on whole blood sample analyses. The linear dynamic range of HbA1c level is from 2.5% to 17% which covers the clinical concentration range with negligible interference from other carbohydrates. One of the main advantages of the boronate affinity-based method is that the measurement does not interfere with Hb variants such as HbF, HbS, HbC, formyl-Hb etc., which makes it applicable to a large population. In addition, boronate-based reagents are relative stable and cost-effective compared to HbA1c antibodies. Due to the nature of the boron-cis-diol covalent bond, this method actually measures GHb (HbA1c and Hb glycated at other Lys sites) and also interferes with other endogenous sugar molecules in the sample. Because of its high specificity and selectivity, the HbA1c antibody is the best candidate for capturing HbA1c. [PERSON] and coworkers developed a micro immunosensor consisting of an ISFET integrated chip (see Section 2.2.3 above) and an electrode array based on the micro-electro-mechanical systems (MEMS) technology, modified with anti-Hb and anti-HbA1c antibodies [51]. The sensor can simultaneously detect the concentration of HbA1c and Hb to obtain HbA1c level. The responses of the immunosensor are linear over the concentration range of 166.7-570 ng/ml Hb and 50-170.5 ng/ml HbA1c which are 10\({}^{5}\)-fold lower than physiological Hb concentrations. To apply the method for whole blood analysis, the samples were lysed and diluted 150,000-fold prior to measurements. This micro immunosensor exhibited a low relative deviation of measured HbA1c level. By reducing cost, enhancing the shelf-life of the antibody, and invoking lab-on-a-chip (LOC) technology, electrochemical immunosensors will have a great potential to be translated into clinical use. [PERSON] and coworkers developed a disposable microfluidic amperometric dual-sensor for the detection of HbA1c and total Hb separately [74]. The concentration of total Hb was measured by the cathodic currents of total Hb catalyzed by a TBO/pTTBA@MWCNT-modified working electrode. On the other surface of the dual-sensor, aptamer was used for HbA1c immobilization. After removal of unbound Hb by washing with PBS, the cathodic current of HbA1c captured on the surface was determined by the aptamer/TBO/pTTBA@ MWCNT-modified working electrode in the fluidic channel.