known as SELEX (systematic evolution of ligands by exponential enrichment) [34,
35]. Aptamers possess high association constants which helped them to bind to
target molecule with high affinity and specificity. Aptamers can be easily synthesized, simply amplified and also the process is easily reproducible. Initially, aptamers are used in the field of oncology, biosensing, cell imaging, drug delivery etc.
[36–39], however, recently those are also used in the detection of different metal
ions (e.g. Cd
2+ , Hg
2+ etc.).
4 Labels
Labels are the molecules conjugated with the biorecognition molecules to generate
a signal. They play important roles in maintaining the sensitivity of the
strip. Different types of labels are used in immunochromatographic assay as given
below.
• Colloidal gold nanoparticles
• Silver nanoparticles
• Quantum dots
• Carbon nanotubes
• Upconverting phosphor nanoparticles
• Liposomes
• Enzymes etc.
However, gold NPs, semiconductor QDs and carbon-based nanoparticles are
mostly used in the ICS-based sensors.
4.1 Colloidal Gold Nanoparticles
These are the most widely used nanoparticles in ICS for signal visualization.
Colloidal gold NPs are chemically stable, easy to synthesize and take short time to
analyze [40–43]. Au NPs can be easily functionalized with different proteins and
oligonucleotides by ionic and covalent interactions. In case of ionic interactions, the
stability of NPs depends on surface charge of the functionalized NPs where electrostatic repulsion force exists among the similarly charged NPs. However, electrostatic attraction force results in aggregation of the AuNPs turning the color from
red to blue [44, 45]. Shape, size and stability of gold nanoparticles affect the
performance of ICS. During the synthesis of Au NPs, size and shape of the particles
can be controlled by the amounts of the reagents i.e. reducing agent, stabilizing
agents, gold-precursors etc. [46]. Gold nanoparticles of size 20–30 nm with strong
absorbance at around 520 nm [46, 47] are generally used as labels here, however,
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M. Pant et al.
35]. Aptamers possess high association constants which helped them to bind to
target molecule with high affinity and specificity. Aptamers can be easily synthesized, simply amplified and also the process is easily reproducible. Initially, aptamers are used in the field of oncology, biosensing, cell imaging, drug delivery etc.
[36–39], however, recently those are also used in the detection of different metal
ions (e.g. Cd
2+ , Hg
2+ etc.).
4 Labels
Labels are the molecules conjugated with the biorecognition molecules to generate
a signal. They play important roles in maintaining the sensitivity of the
strip. Different types of labels are used in immunochromatographic assay as given
below.
• Colloidal gold nanoparticles
• Silver nanoparticles
• Quantum dots
• Carbon nanotubes
• Upconverting phosphor nanoparticles
• Liposomes
• Enzymes etc.
However, gold NPs, semiconductor QDs and carbon-based nanoparticles are
mostly used in the ICS-based sensors.
4.1 Colloidal Gold Nanoparticles
These are the most widely used nanoparticles in ICS for signal visualization.
Colloidal gold NPs are chemically stable, easy to synthesize and take short time to
analyze [40–43]. Au NPs can be easily functionalized with different proteins and
oligonucleotides by ionic and covalent interactions. In case of ionic interactions, the
stability of NPs depends on surface charge of the functionalized NPs where electrostatic repulsion force exists among the similarly charged NPs. However, electrostatic attraction force results in aggregation of the AuNPs turning the color from
red to blue [44, 45]. Shape, size and stability of gold nanoparticles affect the
performance of ICS. During the synthesis of Au NPs, size and shape of the particles
can be controlled by the amounts of the reagents i.e. reducing agent, stabilizing
agents, gold-precursors etc. [46]. Gold nanoparticles of size 20–30 nm with strong
absorbance at around 520 nm [46, 47] are generally used as labels here, however,
242
M. Pant et al.