On-bead sensor

Andersson, H., van der Wijngaart, W., Enoksson, P., Stemme, G., Micromachined flow-through filter-chamber for chemical reactions on beads, Sensors Actuators B 2000, 67, 203-208. 

Figure 14.2. On-bead sensors, (a) Metal sensors using a dansyl fluorophore following a PET mechanism, (b) Schematic representation of a peptide-based FRET sensor to monitor protease activity. Adapted with permission from Refs 8 (left) and 15 (right). Copyright 2000-2009 American Chemical Society. (See the color version of this figure in Color Plates section.)...

On-bead sensors have also been developed on the basis of the fluorescence energy transfer (FRET) mechanism (Figure 14.2b). FRET signaling is less stringent than PET in terms of binding domain structure but requires large changes in the distance between donor and acceptor, which results in their main application within peptide or nucleic acid structures. On-bead FRET sensors constructed by split-and-mix techniques were initially... 

The avidin-biotin interaction has also been used to immobilize antibodies and proteins, especially in commercial systems based on surface plasmon resonance (SPR) measurements (e.g., the BIAcore). The extraordinary affinity (Kl 10-15 M) of avidin (or its bacterial relative, streptavidin) for the vitamin biotin is the basis of this immobilization procedure. A solid support (e.g., glass beads, sensor chip, optical fiber) covered with avidin can be used as an activated carrier for a very sturdy immobilization of previously biotinylated antibodies. In spite of the many methods for biotinylating proteins described in the literature, the use of biotinyl N-hydroxysuccinimide ester (BNHS) and similar derivatives, remains the most useful [65]. 

We synthesized lOmer and 15mer peptides, which included the amino acid sequence of the phosphorylated site, ArgArgAlaSerLeu, of PKA. Ser is phosphorylated by PKA in the cell. EDANS-modified lOmer and 15mer peptides were synthesized on bead. After cleavage and deprotection of peptide side chain, dabcyl group was attached at the N-terminal a-amino group. Ser-phosphorylated sensor peptides were also synthesized. The synthesized sensor peptides were purified using HPLC and identified by MALDI-TOF MS (Table 1). 

RNA aptamer specifically binding and inhibiting the activity of the catalytic ricin A-chain (RTA) has been selected [57]. Initially 80 nucleotides long, it has been shortened to 31-nucleotide aptamer that contained all sequences and structures necessary for interacting with RTA. Inhibition properties of this aptamer exceeded known ricin inhibitors. This aptamer along with others was used to develop a multispecific sensor. Aptamers were immobilized on beads, introduced into micromachined chips on the electronic tongue sensor array, and used for the detection and quantification of proteins. The lowest detected ricin concentration using this sensor was 1 pM (320 ng/mL) [58]. 

Infrared sensors Although these are costlier with respect to catalytic bead sensors they offer a number of advantages such as faster response ( 10 s), low maintenance, self-checking with the help of microprocessor circuitry, and measurement does not depend on O2 to detect the gas. It may be designed in... 

A. Palma, M. Tasior, D. O. Frimannsson, T. T. Vu, R. Meallet-Renault, D. F. O Shea, New on-bead near-infrared fluorophores and fluorescent sensor constructs. Org. Lett. 2009, 11, 3638-3641. 

As it is shown in Figure 8, the enzyme can be immobilized in the vicinity (membrane, beads, etc) or on the surface of the fiber for optical fiber sensor development. Alternatively, it can be placed in a reactor and use the optode... 

Figure 8. Seventy-six sensor beads (Jupiter C4/Nile Red) monitored to show that the average responses for three consecutive 0.38-s exposures of 50% saturated vapor levels result in reproducible and high-speed response profiles. The sensors are positioned on the distal tip of an optical imaging fiber and relative analyte concentrations are 0.5 and 18700 ppm for 1,3-DNB and toluene, respectively. Reprinted with permission from ref 12a. Copyright 2000 American Chemical Society.

S. Centi, S. Laschi, M. Franek, and M. Mascini, A disposable immunomagnetic electrochemical sensor based on functionalized magnetic beads and carbon-based screen-printed electrodes (SPCEs) for the detection of polychlorinated biphenyls (PCBs). Anal. Chim. Acta 538, 205—212 (2005). 

Similar to the work described by Spohn et al. [34], a trienzyme sensor was developed recently for the determination of branched-chain amino acids (L-valine, L-leucine, and L-isoleucine). Leucine dehydrogenase, NADH oxidase, and peroxidase were coimmobilized covalently on tresylate-hydrophylic vinyl polymer beads and packed into a transparent PILL tube (20 cm X 1.0 id), which was used as flow cell. The sensor was free of interferences from protein and NH4+ and it was stable for 2 weeks. The sensor system was applied to the determination of branched-chain amino acids in plasma with recoveries ranging from 98 to 100% [36],... 

CL emission. The system allows a simple determination of phosphate in 3 min with a linear range of 4.8-160 pM. Owing to its sensitivity, this method could be satisfactorily applied to the analysis of maximum permissible phosphate concentrations in natural waters [42-44], Also, the maltose-phosphorylase, mutar-ose, and glucose oxidase (MP-MUT-GOD) reaction system combined with an ARP-luminol reaction system has been used in a highly sensitive CL-FIA sensor [45], In this system, MP-MUT-GOD is immobilized on A-hydroxysuccinimide beads and packed in a column. A linear range of 10 nM-30 pM and a measuring time of 3 min were provided, yielding a limit of detection of 1.0 pM as well as a satisfactory application in the analysis of river water. 

Catalytic gas detection is based on the principal that oxidation of a combustible gas in air is promoted at the surface of a heated catalyst such as a precious metal. The oxidation reaction results in the generation of heat that provides a direct measure of the concentration of the gas that has been reacted. The sensing element embodying the catalyst is a small bead that is supported with the sensor. 

Some bead materials possess porous structure and, therefore, have very high surface to volume ratio. The examples include silica-gel, controlled pore glass, and zeolite beads. These inorganic materials are made use of to design gas sensors. Indicators are usually adsorbed on the surface and the beads are then dispersed in a permeation-selective membrane (usually silicone rubbers). Such sensors possess high sensitivity to oxygen and a fast response in the gas phase but can be rather slow in the aqueous phase since the gas contained in the pores needs to be exchanged. Porous polymeric materials are rarer and have not been used so far in optical nanosensors. 

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