Thus, the OPD technology offers great potential for developing simple, inexpensive, portable, disposable sensor devices for use in the field. Recently, the use of OPDs in integrated microfluidic systems for both fluorescence and chemiluminescence detection of analytes has been reported [6�C9]. A fluorescence detector for biomarkers and molecular probes [7,9] was constructed from heterojunction photodiodes of copper phthalocyanine (CuPc) and fullerene (C60), whilst chemiluminescence sensors [4,6] were developed using blend heterojunctions of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM). However, the insufficient sensitivity and high detection limit (LOD) of the reported detection methods may limit the use of OPD-based analytical systems compared to conventional analytical chips.

In the past few years there has been tremendous progress in the field of OPD technology, and new conjugated polymers and C70 derivatives have been developed with the purpose of improving the power conversion efficiency of organic solar cells [10,11]. Poly[N-9��-heptadecanyl-2,7-carbazole-alt-5,5-(4��,7��-di-2-thienyl-2��,1��,3��-benzothiadiazole)] (PCDTBT) is one of new conjugated polymers that forms a blend heterojunction with PC70BM, resulting in detectors with enhanced light absorption, higher short-circuit current density and lower dark current compared to P3HT:PC60BM blends [12]. Owing to these unique characteristics, the PCDTBT:PC70BM heterojunction photodiode may be promising for OPD-based chemical and biological sensors in applications requiring high detection sensitivities.

Detection of pathogens in water, for instance, often involves analysis for low concentrations of protozoa, bacteria and viruses in the samples. Highly sensitive methods are thus required for monitoring the safety of water [13�C15], and the tests are preferred to be conducted at the point of care in order to obtain rapid results [16]. An ideal point-of-care device should perform multiplexed tests as both drinking water and surface water are vulnerable to Escherichia coli, Campylobacter and adenovirus, among others [17,18]. Although arrays of organic light emitting diodes have been used for multiplexed detection [19], no OPD-based multi-analyte sensor has yet been demonstrated.Multiplexed detection of pathogens can be accomplished using immunological methods [20,21].

Immunoassays are extensively employed in lab-on-a-chip devices, and rapid analyte detection is typically achieved by use of either optical or electrochemical methods [22�C24]. The electrochemical method is discouraged in disposable microfluidic chips as it is often affected Brefeldin_A by variations of temperature, pH and ionic concentrations, whilst the development of optical methods for microfluidic sensors has recently received considerable attention in on-site sensing [2].