An Organic Photoelectrochemical Transistor Biosensor Based On Exciton-plasmon Resonance Energy Transfer

In recent years,organic electrochemical transistors(OECTs)have been widely used for biosensor applications,due to their advantages of low cost,easy fabrication,low working voltage(<1 V),biocompatibility,easy miniaturization,and flexibility.As OECT-based biosensor can serve as both an amplifier and a sensor,normally it has a very high sensitivity.On the other hand,photoelectrochemical(PEC)bioanalysis is gaining increasing attention in the area of analytical chemistry.In PEC detection system,light is utilized to excite the photoactive species and the electrical signal is transduced as the detection readout.Using the two separate forms of signal for excitation and detection,this technique possesses high sensitivity because of the reduced background noise.OECT and PEC technology have many complementary advantages and are supposed to be integrated together.Therefore,in this thesis,we propose a novel biosensor based on organic photoelectrochemical transistor(OPECT),in which the working electrode in PEC system is served as the gate electrode of OECT.The synthesis of materials and the fabrication of OPECT were studied,as well as the new sensing technology was constructed.Moreover,the new biosensors were then used for the detection of DNA and Salmonella.First of all,CdS quantum dots(CdS QDs)and gold nanoparticles(Au NPs)were successfully synthesized and characterized.The results showed that they were uniform in particle size distribution and their dispersion abilities were good.Their sizes were both 5±1 nm.The absorption spectrum showed that the CdS QDs had broad absorption ranges,which were suitable for employment as photoactive species in PEC and OPECT biosensors.It was found that the emission spectra of CdS QDs overlaped with the absorption spectra of Au NPs,which was favorable for the surface plasmon resonance(SPR)of Au NPs.As a result,the exciton-plasmon interaction(EPI)between the two particles was quickly generated.The OPECT devices were fabricated and characterized.The devices showed excellent stability in AA solution.At the same time,the channel current of the devices could be modulated for more than one order of magnitude by a gate voltage of only 0.6 V.The OPECT devices obtained were excellent candidates for the construction of highly sensitive biosensors.Then,a new sensing technique based on OPECT was successfully constructed,in which the working principle was explored.It was found that the photoactive semiconductor materials on the gate electrode undergone an electron transition and a charge transfer at an appropriate photoexcitation,accompanying the generation of photovoltage on the gate electrode.As a result,the potential drop at the gate/electrolyte interface of OPECT device changed,leading to the changes of effective gate voltage and channel current of the device.OPECT-based biosensors were then used for DNA sensing.The label-free OPECT-based DNA sensor could detect complementary target DNA(10 bases)at the concentration of 10 nM,which was mainly due to the hindrance effect of target DNA on the charge transfer on the gate electrode.The detection limit was extended to 1 fM by labeling the target DNA with Au NPs,which was 7 orders of magnitude lower than that of the label-free DNA sensor.In the Au NPs-labeled DNA sensor,the charge transfer on the surface of the gate electrode was greatly reduced by the EPI effect between CdS QDs and Au NPs.In addition,the effect of particle spacing on the EPI effect was studied.It was found that both the steric hindrance effect and the EPI effect exsited in the Au NPs-labeled DNA sensor.At the same time,we found that the detection limit of OPECT-based DNA sensor was 1~2 orders of magnitude lower than that of PEC-based DNA sensor,indicating that OPECT-based biosensors had higher sensitivities.Finally,OPECT-based immunosensor was also successfully developed for the detection of Salmonella.The detection limit of OPECT-based Salmonella sensor was 102 cells/m L,which was due to the hindrance effect of Salmonella on the charge transfer on the gate electrode.Furthermore,Salmonella with a concentration of as low as 10 cells/mL could be detected by labeling the Salmonella with Au NPs,which was one order of magnitude lower than the detection limit of the Salmonella sensor based on the steric hindrance effect.The sensing mechanism could be attributed to the EPI effect between CdS QDs and Au NPs,which greatly reduced the charge transfer on the surface of the gate electrode.This novel universal sensing technique can be supposed to be used for other bioanalysis,such as the detection of virus,marine microbes,and cells.