Finalidad e importancia del acogimiento familiar

The applications of nanoparticles for biosensors have been extensively studied, including fluorescence, colorimetric, electrochemical sensing and so on. Besides, there are numerous works about fabrication of multifunctional platform that combines the feature of drug delivery, bio-targeting and sensing, fluorescence imaging, and even magnetic property all together in single nanoparticle, but will be beyond the content of this chapter.

4.1.1 Fluorescence sensing

Quantum dots are semiconductive nanocrystals with size below 10 nm and high quality fluorescence property. Comparing to conventional organic fluorescent dyes, quantum dots have been famous for the easy excitation, tunable, narrow and symmetric emission spectrum1, which are all preferable character for serving as fluorescence probe in bio-detection. One of the earliest

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reported works of using nanomaterials for bio-sensing purpose was reported in 1998. Both Nie2 and Alivisatos’s3 groups used different color emitting core-shell quantum dots conjugated with selected surface functionality to detect the location of targeted protein through either electrostatic force/hydrogen bond or ligand-receptor interactions such as biotin-avidin and antibody-antigen, and multicolor dye images under single excitation were presented. These studies exhibited the outstanding advantage of quantum dots for fluorescence labeling and detection, also demonstrated the photochemical stability of quantum dot in biological environment. As water solubility being vital for biological application, the as-prepared hydrophobic quantum dot was converted to hydrophilic by silica coating3 or mercaptoacetic acid capping2 to increase bio- compatibility. Soon the research of quantum dot bio-labeling has been developing significantly, majorly focused on the surface functionalization that is adaptable to the target molecules while enhancing biological buffer compatibility of the quantum dots, including ligand exchange4 and amphiphilic polymer encapsulation5 after which further functionality will take place through crosslinkers to complete bio-molecule modification. So far, quantum dot has been conjugated with antibody and peptide for the imaging and detection of virus infection,6 cancer cell5,7,8 and cell membrane proteins.9,10 In addition, ssDNA-quantum dots conjugation has been widely explored for fluorescence resonance energy transfer (FRET) biosensor11-13 in which hybridization of the targeted DNA triggers the fluorescence of quantum dots that is originally quenched.

However, being highly capable of different surface functionalization also presents drawback for bio-sensing application. Since quantum dot is multivalent which means the category and number of linked molecules varies, it is hard to control the stoichiometry, orientation and conformation of the conjugation. So the relationship between analyte concentration and output

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signal strength is difficult to determine. Therefore it is more suitable for qualitative imaging rather than quantitative sensing and analysis.14

4.1.2 Colorimetric sensing

The principle of colorimetric sensor is the recognizable color change of metal nanoparticle, mostly gold or silver, under different aggregation states.15,16 Metal nanoparticles are first linked with either oligonucleotide, aptamer or other organic complex that specifically interact with targeted molecules and then result in aggregation and the change of color can be quantified through spectrophotometer. This sensing approach was first applied to lead ion detection17,18 by using “DNAzyme” which is literally a DNA strand with Pb2+ binding site that cleaves into two parts upon contact with Pb2+. The resulted DNA strands hybridize with complementary sequence from neighboring particles and eventually cause aggregation. Colorimetric cocaine detection was also carried out by using cocaine aptamer linked gold nanoparticles. The nanoparticles were originally aggregated because of the pairing of conjugated oligonucleotide, but became disassembled with the present of cocaine as the aptamers binding to cocaine molecules and releasing the nanoparticles from each other, inducing color change of blue to red, of which the rate of change increased with cocaine concentration in the range of 50 to 500 µM. Recently, the application colorimetric nanoparticle sensors has been widely expanded to the detection of multiple heavy metal and other hazardous ions, including, but not limited to, copper,19 mercury,20 silver,21 cobalt,22 arsenic23 and cyanide24. This nanoparticle based sensor has also been investigated for detection of peptides such as enzyme and biomarker,25 as well as targeted DNA/RNA sequence without amplified through PCR (polymerase chain reaction) for genetic

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diagnosis of decease and pathogen, such as cancerous cell,26,27 Escherichia coli,28 and Salmonella bacteria.29

4.1.3 Electrochemical sensing

Metallic and semiconductive nanoparticles can be immobilized on electrode and perform as electrochemical sensor through the specific interaction between particle surface functionality and analyte that causes conductivity change. The increased surface area comparing to conventional electrodes remarkably enhances the efficiency of electron transfer from analyte and improves detection sensitivity.30-32 Numerous studies have been reported about using nanoparticle electrochemical sensor for the detection of nucleic acid. Mirkin et al33 developed a DNA array detection method started with short oligonucleotide functionalized SiO2 film deposited between two electrodes. Colloidal gold nanoparticles modified with oligonucleotide were then applied and bind to the SiO2 film with the present of target DNA sequence through hybridization since both oligonucleotides were designed to be complementary with part of target DNA. Subsequently, silver deposition on the colloidal gold was introduced, functioned as signal amplifier to achieve detection sensitivity down to 500 fM with a point mutation selectivity of about 100000:1.33 Detections of proteins or peptides have also been realized by using antibody34,35 or aptamer36-38 based on similar principle. Nanoparticle electrochemical sensor is also adapted to enzyme based sensing by immobilizing enzyme molecules on particles for the detection of substrate concentration or enzyme activity. One popular application is test of glucose concentration by using glucose oxidase functionalized particles, including platinum,39 gold40,41, magnetite42 and so on. As redox reaction, the oxidation of glucose triggers electron transfer in the enzyme- nanoparticle complex and the signal is recorded by voltametric analyzer.32

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