Programmed Assembly of Functionalized Nanoparticles

As mentioned earlier one of interesting features of metal nanoparticles is their fascinating optical properties “Hyperlink Missing”. So to control changes in a solution during the production of nanoparticles is to monitor changes in the solution optical properties. 

Nanoparticles for the detection of specific DNA

The nanoparticles shown in following figure are designed to detect the presence of specific DNA strand. For this purpose the nanoparticles are modified via a thiol bond with single stranded DNA complementary to the target DNA. The modification of the nanoparticles affects the amount of light absorbed by the solution, compared to the un-modified nanoparticles, which have a characteristics peak at 560 nm.

When target DNA analyte is added to the solution, aggregation occurs. Nanoparticles move closer together and form clusters. The formation of nanoparticle clusters results in changes of absorbance of the solution. The location of the absorbance peak is an indication of average nanoparticle size. When peak is shifted to higher wavelength in this case from 560nm to 600 nm it means that the average size of the particles in the solution is larger due to the aggregation.  The width of the peak much broader after aggregation. This means a higher distribution of nanoparticle size compared to the bare nanoparticles. The presence of a target analyte can be therefore determined by monitoring the absorbance of the solution.

Monitoring the change in optical properties at specific wavelengths, it is possible to examine the sensing properties of these sensors by applying following procedure:

The signal obtained at 560 nm relates to the DNA molecules and the signal obtained at 700 nm relates to aggregates of DNA with gold nanoparticles.

When the solution is heated to 80°C the target DNS dissolves in solution and the relevant signal at 560 nm increases. This results in decrease in the aggregate peak at 700 nm, as the DNA/Au aggregates dissociate to individual nanoparticles. This process is repeatable when heating and cooling the system. The heating and cooling is also visible by a naked eye by looking at solution color changes. 

In general the DNA-capped gold nanoparticles are promising approach for the detection of DNA strands by controlling solution temperature of DNA-modified nanoparticles. 

 Various examples of nanoparticles used for DNA detection

When sensor meets its target DNA, the color of the drop changes due to aggregation. This system does not require sophisticated equipment for analysis and is appropriate mainly to determine the presence of analyte, not its concentration. Detecting small concentrations of target DNA is more challenging. 

In figure X-B the gold nanoparticles modified with short, single-stranded DNA to indicate the presence of particular DNA sequence that is hybridized on a transparent substrate in a three-component sandwich assay format. At high target DNA concentrations, many nanoparticles are attached to the transparent surface. The high concentration of nanoparticles causes a dramatic change in the color and the substrate looks pink. At lower target concentrations, the attached nanoparticles cannot be visualized with the naked eye. To facilitate visualization of nanoparticles labels that are hybridized to the array surface, planting silver ions on the gold nanoparticles is used for signal amplification. Using silver ions very low coverage of nanoparticles can be visualized by a simple scanner or by using naked eye.

In figure X-C the attachment of nanoparticles to a substrate in a three-component sandwich assay format is detected by the change in scattered light.  A more applicable approach is monitoring the electrical changes of a sensor while the sensing mechanism is similar to the previous examples as seen in figure X-D. The system consist of isolating substrate with two electrodes for measuring the electrical resistance. On the top of isolating substrate the nanoparticles are assembled in a low concentration so there is no direct connection between the nanoparticles and therefore the electrical resistance is very high. If DNA is captured, silver will plate the nanoparticles. This plating will contact between the nanoparticles and the electrical resistance will dramatically decrease. By measuring the changes in the electrical resistance we can obtain and monitor sensing signal.

There are different approaches that utilize similar sensing mechanisms, but use different characterization methods that have been developed and tested to detect the attachment of nanoparticles to a substrate.

Monitor the surface enhanced Raman spectroscopy (SERS) signal as seen in figure X-E

Monitor the diffraction of laser beam, when the laser beam is directed towards the nanoparticles  that are attached to the surface as seen in figure X-F

  Magnetic nanoparticles 

To detect tiny amounts of targeted molecules inside the human body or in complicated environments relies on magnetic nanoparticles. Magnetic micro particle probes with antibodies that specifically bind a target of interest and nanoparticle probes that are encoded with DNA that is unique to the protein target of interest and antibodies that can sandwich the target captured by the micro particle probes.

Magnetic separation of the complex probes and target is followed by de-hybridization of the DNA on the nanoparticle probe surface, thus allowing one to determine the presence of the target protein by identifying the DNA sequence released from the nanoparticle probe.