In recent years, the population of chronic kidney disease (CKD) in the world has increased rapidly due to the coming of aging society. The risk of death and the need of patient care for CKD patients is considerably high, so it is expected that as the number of CKD patients increases, the cost and burden on medical resources will also be inevitably high . Therefore, CKD screening especially for early detection is critical for the saving of medical resources. Urea, generated from protein breakdown, is the principle nitrogenous waste product of metabolism, which is eliminated from the body almost exclusively by the kidneys in urine . Since CKD is associated with reduced urea excretion, CKD patients consequently have higher urea nitrogen levels. Therefore, to determine the level of nitrogen in the body and to evaluate the adequacy of dialysis treatment, a blood test has been typically performed to acquire blood urea nitrogen (BUN) level. However, the invasive test and its analysis can be only conducted in medical center, which is accurate but lack of immediacy. Therefore, developing a low-cost, portable, real-time, and non-invasive monitoring device for CKD screening is useful especially for the rural areas where medical resources are scarce. Violate organic compounds (VOCs) contained in exhaled breath gases have been proposed as biomarkers for preliminary diagnosis of human diseases such as lung cancer and diabetes. Many researchers have also reported that breath ammonia (breath-NH3) has strong correlation to BUN levels. In our previous work, we collected the pre- and post-dialysis breath ammonia concentration and the BUN levels of 40 hemodialysis patients. The result showed that the correlation coefficient between breath ammonia and BUN was 0.82, indicating a high degree of positive correlation. Therefore, breath ammonia has been proposed as a 

promising biomarker of CKD, which can be used to reflect the kidney function.

There have been several methods developed for the measurement of exhaled VOCs, including gas chromatography-mass spectrometry (GC-MS), proton-transfer reaction-mass spectrometry (PTR-MS), selected ion flow tube-mass spectrometry (SIFT-MS), and laser spectroscopy. GC-MS, PTR-MS, and SIFT-MS are highly sensitive and accurate, but the instruments are bulky and expensive. Laser spectroscopy has the advantage of being portable, but its operation is still complicated, which is not convenient for clinical use. Since the concentration of breath ammonia is in the level of hundreds of ppb, highly sensitive organic semiconductor-based gas sensors have been developed to achieve ppb-regime sensing capability and to provide real-time detection in a portable, easy-to-use, and cost-effective way. However, the operational current of those gas sensors is only in the range of sub-micron ampere or even much lower, which also raises the requirement of instrumentation. Table 1 shows the characteristics of reported ammonia gas sensor, and it demonstrate both the issue of the low operational current and relative high detection limit which may not be suitable for the application of breath detection. Therefore, we have proposed delicate design of device architectures, such as vertical nano-junction (VNJ) and double sensing layer structure [32], to raise the current level up to the order of 10−6 A along with low detection limit to the order of ppb region.

 

We demonstrate a (poly[(4,8-bis(2-ethylhexyloxy)-benzo-(1,2-b:4,5-b)dithiophene)− 2,6-diyl-alt-(4-(2-ethylhexyl)− 3-fluorothieno[3,4-b]thiophene-)− 2-carboxylate-2,6-diyl)]) (PTB7) based ammonia gas sensor with high operational current up to 10−4 A. Using (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) (F4-TCNQ) as a p-type dopant for PTB7, the operation current under driving voltage of 5 V was effectively increased to 1.5×10−4 A, while that of a standard device was only 3×10−6 A. According to the study of space-charge-limited current and ultraviolet photoemission spectroscopy (UPS), the enhanced operational current can be attributed both to the higher mobility and lower carrier injection barrier of the F4-TCNQ doped PTB7. Detailed comparative analysis between the F4-TCNQ doped sensor and the non-doped sensor was also studied. The increased operational current and the current drop during sensing of the F4-TCNQ doped sensors are both beneficial to the realization of low-cost circuitry sensing system, which demonstrates the potential of the strategy of utilizing p-type doping in organic gas sensors for the developing of affordable point-to-care (POC) devices.