7.2 Quantum dots and nanoparticles in sensing applications

Quantum dots and nanoparticles are tiny powerhouses in sensing applications. These nanoscale materials have unique optical and electronic properties that make them perfect for detecting chemicals, biomolecules, and environmental changes with high sensitivity and specificity.

By tweaking their size, shape, and surface chemistry, scientists can create sensors that light up, change color, or emit signals when they detect specific targets. This versatility opens up a world of possibilities for medical diagnostics, environmental monitoring, and more.

Quantum Dot and Nanoparticle Properties

Quantum Dots: Nanoscale Semiconductors

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• Quantum dots are nanoscale semiconductor crystals with unique optical and electronic properties
• Typically range in size from 2-10 nanometers in diameter
• Composed of elements from groups II-VI (CdSe, CdS, ZnSe), III-V (InP, InAs), or IV-VI (PbS, PbSe) of the periodic table
• Exhibit quantum confinement effects due to their small size, which leads to discrete energy levels and size-dependent properties

Size-Dependent Properties and Quantum Confinement

• The optical and electronic properties of quantum dots are strongly dependent on their size and shape
• As the size of the quantum dot decreases, the bandgap energy increases, leading to a blue shift in the absorption and emission spectra
• Quantum confinement occurs when the size of the quantum dot is smaller than the exciton Bohr radius, resulting in discrete energy levels
• The confinement of electrons and holes in quantum dots leads to enhanced optical properties, such as high quantum yields and narrow emission linewidths

Fluorescence and Surface Plasmon Resonance in Nanoparticles

• Nanoparticles, such as gold and silver nanoparticles, exhibit unique optical properties due to their small size
• Gold and silver nanoparticles can exhibit surface plasmon resonance (SPR), which is the collective oscillation of conduction electrons in response to an external electromagnetic field
• SPR in nanoparticles leads to enhanced absorption and scattering of light at specific wavelengths, depending on the size, shape, and composition of the nanoparticles
• Quantum dots exhibit strong fluorescence due to their high quantum yields and narrow emission linewidths
• The fluorescence emission wavelength of quantum dots can be tuned by changing their size, enabling multiplexed sensing applications

Functionalization and Bioconjugation

Surface Modification and Bioconjugation Strategies

• Functionalization of quantum dots and nanoparticles involves modifying their surface to improve their stability, biocompatibility, and targeting capabilities
• Surface modification can be achieved through the attachment of functional groups, such as carboxyl (-COOH), amine (-NH2), or thiol (-SH) groups
• Bioconjugation involves the attachment of biomolecules, such as antibodies, peptides, or nucleic acids, to the surface of quantum dots or nanoparticles
• Common bioconjugation strategies include covalent coupling, electrostatic interactions, and streptavidin-biotin interactions

Targeting Ligands and Biomolecule Attachment

• Targeting ligands, such as antibodies or aptamers, can be attached to the surface of quantum dots or nanoparticles to enable specific binding to target molecules or cells
• Antibodies can be attached to quantum dots or nanoparticles through covalent coupling methods, such as carbodiimide chemistry or maleimide-thiol coupling
• Aptamers, which are single-stranded DNA or RNA molecules that can bind specifically to target molecules, can be attached to quantum dots or nanoparticles through biotin-streptavidin interactions
• The attachment of enzymes or other functional proteins to quantum dots or nanoparticles can enable the development of biosensors for the detection of specific analytes

Sensing Applications

Optical and Chemical Sensors

• Quantum dots and nanoparticles can be used as optical sensors due to their unique optical properties and sensitivity to changes in their local environment
• Changes in the fluorescence intensity, wavelength, or lifetime of quantum dots can be used to detect the presence of specific analytes or changes in pH, temperature, or other environmental conditions
• Gold and silver nanoparticles can be used as colorimetric sensors based on changes in their SPR properties upon binding to target molecules
• Quantum dots and nanoparticles can be functionalized with molecular recognition elements, such as antibodies or aptamers, to create highly selective chemical sensors

Biosensors and Multiplexed Sensing

• Quantum dots and nanoparticles can be used to develop highly sensitive and selective biosensors for the detection of biomolecules, such as proteins, nucleic acids, or small molecules
• The unique optical properties of quantum dots, such as their narrow emission linewidths and resistance to photobleaching, make them ideal for multiplexed sensing applications
• Multiplexed sensing involves the simultaneous detection of multiple analytes using quantum dots with different emission wavelengths
• Quantum dot-based biosensors have been developed for the detection of cancer biomarkers, infectious diseases, and environmental pollutants
• Nanoparticle-based biosensors, such as those using gold or magnetic nanoparticles, have been used for the detection of DNA, proteins, and other biomolecules with high sensitivity and specificity