Vellaiappillai Tamilavan

Optimization of Phosphatase- and Redox Cycling-Based Immunosensors and Its Application to Ultrasensitive Detection of Troponin I

The authors herein report optimized conditions for ultrasensitive phosphatase-based immunosensors (using redox cycling by a reducing agent) that can be simply prepared and readily applied to microfabricated electrodes. The optimized conditions were applied to the ultrasensitive detection of cardiac troponin I in human serum. The preparation of an immunosensing layer was based on passive adsorption of avidin (in carbonate buffer (pH 9.6)) onto indium-tin oxide (ITO) electrodes. The immunosensing layer allows very low levels of nonspecific binding of proteins. The optimum conditions for the enzymatic reaction were investigated in terms of the type of buffer solution, temperature, and concentration of MgCl(2), and the optimum conditions for antigen-antibody binding were determined in terms of incubation time, temperature, and concentration of phosphatase-conjugated IgG. Very importantly, the antigen-antibody binding at 4 °C is extremely important in obtaining reproducible results. Among the four phosphatase substrates (L-ascorbic acid 2-phosphate (AAP), 4-aminophenyl phosphate, 1-naphthyl phosphate, 4-amino-1-naphthyl phosphate) and four phosphatase products (L-ascorbic acid (AA), 4-aminophenol, 1-naphthol, 4-amino-1-naphthol), AAP and AA meet the requirements most for obtaining easy dissolution and high signal-to-background ratios. More importantly, fast AA electrooxidation at the ITO electrodes does not require modification with any electrocatalyst or electron mediator. Furthermore, tris(2-carboxyethyl)phosphine (TCEP) as a reducing agent allows fast redox cycling, along with very low anodic currents at the ITO electrodes. Under these optimized conditions, the detection limit of an immunosensor for troponin I obtained without redox cycling of AA by TCEP is ca. 100 fg/mL, and with redox cycling it is ca. 10 fg/mL. A detection limit of 10 fg/mL was also obtained even when an immunosensing layer was simply formed on a micropatterned ITO electrode. From a practical point of view, it is of great importance that ultralow detection limits can be obtained with simply prepared enzyme-based immunosensors.

Hydroquinone Diphosphate as a Phosphatase Substrate in Enzymatic Amplification Combined with Electrochemical–Chemical–Chemical Redox Cycling for the Detection of E. coli O157:H7

Signal amplification by enzyme labels in enzyme-linked immunosorbent assays (ELISAs) is not sufficient for detecting a low number of bacterial pathogens. It is useful to employ approaches that involve multiple signal amplification such as enzymatic amplification plus redox cycling. An advantageous combination of an enzyme product [for fast electrochemical-chemical-chemical (ECC) redox cycling that involves the product] and an enzyme substrate (for slow side reactions and ECC redox cycling that involve the substrate) has been developed to obtain a low detection limit for E. coli O157:H7 in an electrochemical ELISA that employs redox cycling. In our search for an alkaline phosphatase substrate/product couple that is better than the most common couple of 4-aminophenyl phosphate (APP)/4-aminophenol (AP), we compared five couples: APP/AP, hydroquinone diphosphate (HQDP)/hydroquinone (HQ), L-ascorbic acid 2-phosphate/L-ascorbic acid, 4-amino-1-naphthyl phosphate/4-amino-1-naphthol, and 1-naphthyl phosphate/1-naphthol. In particular, we examined signal-to-background ratios in ECC redox cycling using Ru(NH(3))(6)(3+) and tris(2-carboxyethyl)phosphine as an oxidant and a reductant, respectively. The ECC redox cycling that involves HQ is faster than the cycling that involves AP, whereas the side reactions and ECC redox cycling that involve HQDP are negligible compared to the APP case. These results seem to be due to the fact that the formal potential of HQ is lower than that of AP and that the formal potential of HQDP is higher than that of APP. Enzymatic amplification plus ECC redox cycling based on a HQDP/HQ couple allows us to detect E. coli O157:H7 in a wide range of concentrations from 10(3) to 10(8) colony-forming units/mL.

Highly efficient imide functionalized pyrrolo[3,4-c]pyrrole-1,3-dione-based random copolymer containing thieno[3,4-c]pyrrole-4,6-dione and benzodithiophene for simple structured polymer solar cells

As an effort to improve the photovoltaic properties of a highly efficient large band gap (2.11 eV) alternating copolymer, P(BDT-TDPPDT), comprised of electron rich benzodithiophene (BDT) and novel electron accepting pyrrole-based imide functionalized 4,6-bis(thiophen-2-yl)-2,5-dioctylpyrrolo[3,4-c]pyrrole-1,3-dione (TDPPDT) derivatives, we incorporated a relatively strong electron accepting thiophene-based imide functionalized thieno[3,4-c]pyrrole-4,6-dione (TPD) unit in its main chain via random copolymerization between BDT, TDPPDT and TPD units to give polymer P1. The incorporation of a TPD unit resulted in significant improvement in the optoelectrical and photovoltaic properties. P1 exhibits lower optical band gap (1.91 eV) and a deeper lowest unoccupied molecular orbital (LUMO) energy level compared to those of P(BDT-TDPPDT). The hole mobility of P1 was 3.66 × 10−4 cm2 V−1 s−1 and the PSC made with a simple device structure of ITO/PEDOT:PSS/P1:PC70BM(1 : 2.25 wt%) + 3 vol%/Al gave a maximum power conversion efficiency (PCE) of 7.03% with high photovoltaic parameters, such as an open-circuit voltage (Voc) of 0.87 V, a short-circuit current (Jsc) of 11.52 mA cm−2 and a fill factor (FF) of 70%. Interestingly, P1-based PSCs exhibited a high incident photon to current efficiency (IPCE) of a maximum of 78% at 410 nm and a more than 70% response between 370–590 nm. The PCE achieved in this study is the highest value reported thus far among PSCs made with random copolymers.

Synthesis of new broad absorption low band gap random copolymers for bulk heterojunction solar cell applications

AbstractThree new low band gap random copolymers containing 3-octylthiophene, 2,1,3-benzothiadiazole and 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole at different ratios were synthesized by Suzuki polycondensation. The three copolymers were found to exhibit quite different absorption behaviors. Among the three copolymers, PTTPTTB-P1 prepared with the three monomer units at a ratio of 2:1:1 was found to show a quite broad and flat absorption maximum from 440 to 560 nm as a thin film, whereas PTTPTTB-P2 or PTTPTTB-P3 prepared with the three monomer units at a ratio of 3:2:1 or 3:1:2, respectively, were found to show a distinct absorption maximum at 550 nm or 470 nm, respectively. The optical band gaps of the copolymers estimated from the onset wavelength of absorption were 1.80 eV for all three polymers. Each of the copolymers was examined as an electron donor blending with PC70BM as an electron acceptor in bulk heterojunction (BHJ) solar cells. The maximum power conversion efficiency (PCE) of the BHJ solar cells fabricated in ITO/PEDOT:PSS/polymer:PC70BM (1:5 wt%)/LiF/Al configurations was 2.03% when the PTTPTTB-P1:PC70BM (1:5 wt%) blend was used as an active layer.

Tuning the physical properties of pyrrolo[3,4-c]pyrrole-1,3-dione-based highly efficient large band gap polymers via the chemical modification on the polymer backbone for polymer solar cells

A systematic modulation of the photo-physical properties of high energy converting large band gap (2.04 eV) alternating polymers (PBDTT–DPPD) containing electron rich 2D-conjugated benzodithiophene (BDTT) and weak electron accepting pyrrolo[3,4-c]pyrrole-1,3-dione (DPPD) derivatives via the incorporation of a relatively strong electron accepting thieno[3,4-c]pyrrole-4,6-dione (TPD), thieno[3,4-b]thiophene (TT), or pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) unit on the polymer backbone was demonstrated. All three new random copolymers, RP1, RP2 and RP3, displayed broad absorption bands and lower optical band gaps compared to those of their parent alternating polymer, PBDTT–DPPD. The estimated band gaps of RP1, RP2 and RP3 decreased gradually from 2.04 eV for PBDTT–DPPD to 1.87 eV, 1.60 eV and 1.45 eV, respectively. The decrease in the band gaps of RP1, RP2 and RP3 was associated mainly with the alteration of their conduction bands. Interestingly, RP1 and RP2 showed slightly improved hole mobilities and RP3 exhibited one order lower hole mobility than that of PBDTT–DPPD. The estimated mobilities of RP1, RP2 and RP3 were 1.4 × 10−3 cm2 V−1 s−1, 3.7 × 10−3 cm2 V−1 s−1 and 4.9 × 10−4 cm2 V−1 s−1, respectively. The polymer solar cells (PSCs) prepared from RP1, RP2 or RP3 as donors and PC70BM as an acceptor using a simple device configuration of ITO/PEDOT:PSS/polymer:PC70BM + DIO/Al exhibited a maximum power conversion efficiency (PCE) of 5.35%, 5.05% and 2.41%, respectively.

Property modulation of benzodithiophene-based polymers via the incorporation of a covalently bonded novel 2,1,3-benzothiadiazole-1,2,4-oxadiazole derivative in their main chain for polymer solar cells

Two new electron accepting monomers (BBOB and BOB) containing two serially connected different electron deficient units, such as 2,1,3-benzothiadiazole and 1,2,4-oxadiazole, were prepared and copolymerized with electron-rich benzodithiophene (BDT) derivative to afford polymers P(BDT-BBOB) and P(BDT-BOB), respectively. The optical band gaps of P(BDT-BBOB) and P(BDT-BOB) are calculated to be 2.32 eV and 1.99 eV, respectively, and their highest occupied molecular energy levels are determined to be −5.31 eV and −5.27 eV, respectively. Each of the newly synthesized polymers, i.e.P(BDT-BBOB) and P(BDT-BOB), is used as an electron donor, along with PC61BM as an electron acceptor, in the preparation of polymer solar cells (PSCs). The PSCs made with the configuration of ITO/PEDOT:PSS/P(BDT-BBOB) or P(BDT-BOB):PC61BM (1 : 2 wt%)/LiF/Al gave a maximum power conversion efficiency (PCE) of 1.76% and 2.46%, respectively, and the device performance was further improved to 3.31% and 4.21%, respectively, by simply treating the photoactive layer of PSCs with isopropyl alcohol. Overall, the opto-electrical and photovoltaic properties of the two polymers are found to be quite dependent on the configuration of the covalently bonded 2,1,3-benzothiadiazole and 1,2,4-oxadiazole units incorporated in the polymer main chain.

A fluorescent chiral chemosensor for the recognition of the two enantiomers of chiral carboxylates.

A new 7-nitrobenz-2-oxa-1,3-diazole (NBD)-based fluorescent chiral chemosensor (NBD-1) was prepared and applied to the recognition of the two enantiomers of the tetrabutylammonium salts of N-t-Boc-α-amino acids and chiral carboxylic acids including naproxen. In particular, the chiral recognition by the new fluorescent chiral chemosensor for the two enantiomers of N-t-Boc-threonine (tetrabutylammonium salt) was quite excellent, the Stern-Volmer constant ratio (K(D)/K(L)) for the two enantiomers being as high as 4.89.

Effects of the incorporation of bithiophene instead of thiophene between the pyrrolo[3,4-c]pyrrole-1,3-dione units of a bis(pyrrolo[3,4-c]pyrrole-1,3-dione)-based polymer for polymer solar cells

A new wide band gap polymer, P(BDTT–BTBDPPD), consisting of electron rich 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTT) and electron deficient bithiophene-incorporated bis(pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (BTBDPPD) derivative was prepared to improve the photovoltaic performances of a reported polymer, P(BDTT–TBDPPD), containing BDTT and thiophene-incorporated bis(pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (TBDPPD) derivative. Polymer P(BDTT–BTBDPPD) exhibited maximum absorption at 478 nm and the calculated optical band gap was 2.10 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of P(BDTT–BTBDPPD) were estimated to be −5.44 eV and −3.34 eV. The hole mobility of P(BDTT–BTBDPPD) was 3.22 × 10−4 cm2 V−1 s−1. The polymer solar cells (PSCs) prepared using P(BDTT–BTBDPPD) : PC70BM (1 : 2 wt%) + 3 vol% DIO blend offered a maximum power conversion efficiency (PCE) of 4.62% with an open-circuit voltage (Voc) of 0.90 V, a short-circuit current (Jsc) of 7.99 mA cm−2, and a fill factor (FF) of 64%. This study suggests that the replacement of the thiophene spacer unit located between the pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione units of bis(pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) derivative with a bithiophene unit did not considerably alter the energy levels and charge transport properties of the resulting polymer. However, the overall photovoltaic performance was improved due mainly to the enhanced morphology of the photoactive layer.

Modulation of the properties of pyrrolo[3,4-c]pyrrole-1,4-dione based polymers containing 2,5-di(2-thienyl)pyrrole derivatives with different substitutions on the pyrrole unit

In this study, four new pyrrolo[3,4-c]pyrrole-1,4-dione (DKPP)-based polymers, P(DKPP-TPTH), P(DKPP-TPTE), P(DKPP-TPTA), and P(DKPP-TPTI), containing N-alkyl-2,5-di(2-thienyl)pyrrole (TPT) derivatives with four different substituents such as hydrogen, ester, amide, and imide groups on the 3,4-position of the pyrrole unit were prepared to tune the properties of the polymers. Opto-electrical studies showed that the incorporation of electron withdrawing substituents such as ester, amide and imide groups instead of hydrogen into the pyrrole backbone of the polymers increased the band gaps significantly from 1.31 eV to 1.42 eV, 1.37 eV and 1.37 eV, respectively, and reduced the highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO) energy levels from −4.96 eV/−3.65 eV to −5.24 eV/−3.82 eV, −5.17 eV/−3.80 eV and −5.35 eV/−3.98 eV, respectively. Organic field effect transistors (OFETs) made from these polymers indicated that the incorporation of electron withdrawing functional groups into the polymer backbone reduced hole mobility. Polymer solar cells (PSCs) prepared using polymers as electron donors offered higher power conversion efficiency (PCE) for the polymer containing hydrogen on the TPT backbone, but the polymers incorporating electron withdrawing substituents into the TPT backbone showed a significantly higher open-circuit voltage (Voc) though the PCE was relatively low.

Pyrrole N-alkyl side chain effects on the properties of pyrrolo[3,4-c]pyrrole-1,3-dione-based polymers for polymer solar cells

In this study, two new pyrrolo[3,4-c]pyrrole-1,3-dione (PPD)-based polymers (P3 and P4) incorporating a 2-octyldodecyl (branched alkyl) group on the pyrrole nitrogen of the PPD unit were prepared. Their properties were briefly compared to those of the structurally quite similar PPD-based polymers (P1 and P2) with an n-octyl (linear alkyl) group on the PPD unit, in order to understand the importance of the pyrrole N-alkyl group. The calculated optical band gaps (Eg) and highest occupied molecular orbital energy levels of P3 and P4 were found to be around ∼0.1 eV higher and deeper, respectively, compared to those of the respective linear alkylated polymers P1 and P2. The maximum power conversion efficiency (PCE) obtained for the polymer solar cells made by using P3 or P4:PC70BM blends without any additive was around ∼2%, which was quite similar to that of the PSCs made using P1 or P2:PC70BM blends. However, P3 and P4 exhibited a notably lower PCE than that of P1 and P2, respectively, when the polymer solar cells were created with an additive. This study confirmed that the alkyl substituent on the pyrrole nitrogen of the PPD unit significantly affects the properties of the resulting polymer.