I was gone for few weeks in August on vacation and on coming back I find a bunch of papers all in JACS all using some combination of DNA and gold nanoparticles or gold thin films for design of sensing devices. Gold nanoparticles are my favorite topic but I just can’t seem to keep track of all of them-they are everywhere. Inspite of tremendous research the most common commercial sensing device using gold nanoparticle are still dipstick immunoassays introduced decades ago. I wonder why? What is keeping the new technology from entering into the market? Any thoughts? Drop me an e-mail.
Anyway, here is the summary of what I found.
This technology uses standard micro-fabrication technology to create a nanogap structure with small DNA strand attached to two electrodes. On hybridization with a complementary DNA strand the nanogap is bridged. The bridge is made conducting by depositing silver ions followed by reduction with hydroquinone giving a silver nanowire. The detection limit for this sensor is down to 1.0fM. The technology uses clever electrochemical stripping method for immobilizing different strands of DNA on top and bottom electrodes.
Chad Mirkin pioneered the sensor for DNA hybridization by combining gold nanoparticle modified with DNA molecule with silver enhancement. The trick for successful implementation of this method is stringent washing condition for accurate detection of SNPs. This paper uses ligases to permanently connect capture and the probe strand followed by silver enhancement for accurate detection using naked eye or conventional flatbed scanner. The sensitivity as shown is ~1pM lower by almost 3 order of magnitude reported in previous report.
This report using photon burst technology to develop homogenous DNA hybridization and immunoassay. Photon burst intensity of gold nanoparticles is tens to hundreds time stronger than quantum dots. and 36nm particles could be detected down to 317fM concentration using this method. Affinity molecules (DNA or antibodies) are attached to gold nanoparticles and in presence of analyte the gold nanoparticles aggregate and intensity of photon burst from single gold nanoparticle decreases proportionally. Using this trick the sensing platform can detect DNA concentration down to few femtomolar concentration and protein analytes in picomolar concentration.
This technology leverages the fact that single stranded DNA tethered to a small molecule is protected from degradation by Exonuclease I enzyme (Exo I) when small molecule is attached to its protein target. Small molecule attached DNA is wrapped around a single walled carbon nanotube (SWNT) hence preventing its attachment to a gold layer coated with negatively charged self assembled monolayer (SAMs). On addition of Exo I the ssDNA is degraded and SWNTs attaches to SAMs and current is generated. In presence of specific Protein small molecule interaction the degradation of ssDNA by Exo I is prevented and no current is generated. When folate is attached to ssDNA then folate receptor down to 0.01nanomolar could be detected using this sensing method.