Fluorescent nanoparticles, and their subclass of quantum dots, have been explored for many potential applications including: high efficiency solar panels, LEDs (light emitting diodes), flexible and brighter displays, advanced bioimaging, and biosensing techniques. Most of these potential applications utilize nanoparticles that are unstable in environmental conditions thus requiring sealed (from air and water) systems and careful treatment to avoid oxidation and deterioration. An additional problem with current nanoparticles is that they are made with toxic metals such as cadmium, selenium, lead, or tellurium. The combination of toxicity and instability limit potential nanoparticle uses outside of a laboratory environment.
Nanoparticles characterized as quantum dots are defined as particles that have a radius of less than 100 nanometers. They can be as small as 2 to 10 nanometers, corresponding to 10 to 50 atoms in diameter and a total of 100 to 100,000 atoms within the volume of a quantum dot. A quantum dot confines the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. As a result, these particles exhibit optical and thermal properties which are different from those of the bulk material from which they are made. Quantum dots can show strong quantum confinement effects; they exhibit an inherent fluorescence color—they emit a particular color upon being illuminated by UV light—based on their energy band gap which is controlled by the crystal size and chemical composition. For instance, CdSe covers the whole visible range: the 2 nm diameter CdSe quantum dot emits in the blue range and a 10 nm diameter CdSe quantum dot emits in the red range. The ability to tune the emission spectrum of these nanoparticles throughout the visible region gives researchers an ability to customize the molecules to fit their application.
Toxicity and environmental stability of nanoparticles are particularly important for biological applications such as the detection of tumors and other medical related biosensing applications. Nanoparticles made from zinc, silver and indium have been suggested for these applications (Subramaniam, P., et al., “Generation of a Library of Non-Toxic nanoparticles for Cellular Imaging and siRNA Delivery”, Advanced Materials, 2012, 24, 4014-4019); however, attempts to duplicate this disclosed synthesis and confirm the resulting particles have been unsuccessful. Other work has also demonstrated coupling between CdSe—CdS core-shell quantum dots, enclosed in a silica shell, and biological molecules (Brunchez et al., “Semiconductor nanocrystals as fluorescent biological labels”, Science, 281: 2013-2016 (1998)). Similarly, highly fluorescent nanoparticles (zinc sulfide-capped cadmium selenide) have been covalently coupled to biomolecules for use in ultrasensitive biological detection (Warren and Nie (1998) Science, 281: 2016-2018).
Other uses for nanoparticles include other sensing applications—such as metal detection in water—as their fluorescence intensity has been shown to depend on environmental conditions. The ability to identify contamination in a variety of water sources quickly and inexpensively would greatly help in many different circumstances. Metal contamination in storm water runoff and near shipyards is of great interest for protection of our environment. These types of contamination occur at discrete time points with a limited window to identify the problem before the sample is diluted into the main water stream. If a quick, easy analytical method to indicate if contaminated water may have been released into streams or oceans existed, then more frequent testing could be performed in-situ and many pollution problems could be mitigated.
Currently, the state of the art technique for metal detection in water is inductively coupled plasma mass spectrometry (ICP-MS) which requires samples to be gathered and sent to a laboratory for testing. Although a very accurate and quantitative method, this technique has several drawbacks. The largest drawback being the size and expense of the instrument itself. ICP-MS is not a field portable technique and therefore samples must be collected and transferred back to the laboratory for analysis, a very time consuming task. Samples must also be free from particulates to avoid disrupting flow or blocking the nebulizer. Additionally, continuously running samples with high salt concentrations (like seawater) can eventually lead to blockages. These blockages can be avoided by diluting samples but this begins to affect detection limits and takes time and careful laboratory work.
In contrast to the complicated ICP-MS technique, the presence of metal ions in solution has been shown to influence nanoparticle fluorescence either through a quenching or an enhancement of the nanoparticle fluorescence and thus a potential method of testing. There are several proposed mechanisms for these interactions but the most common mechanism stems from an interaction of the metal ion with a specialized ligand to create a new complex that influences the emission. Specifically, the ligand may recombine with the metal ion leaving behind a surface defect on the nanoparticle which leads to quenching of fluorescence. This quenching process allows for a visual confirmation that a metal ion is present.
These types of quenching interactions between nanoparticles and metal ions have been shown in several types of nanoparticle systems. The most common of these systems are made with toxic materials such as cadmium and either tellurium or selenium. Currently, the most commonly used materials for metal detection applications are lead sulfide, cadmium sulfide, lead selenide, and cadmium selenide. They also frequently use thiol containing ligands such as glutathione (GSH), L-Cysteine (Cys), mercaptoacetic acid (MAA), mercaptopropionic acid (MPA), or mercaptosuccinic acid (MSA) which aide in solubility as well as metal ion affinity. Most of these systems utilize ligands for specific binding of metals to nanoparticles such as using a thiol containing ligand to bind mercury. J. Ke, X. Li, Q. Zhao, Y. Hou, J. Chen, “Ultrasensitive nanoparticle Fluorescence quenching Assay for Selective Detection of Mercury Ions in Drinking Water”, Sci. Rep. 2014, 4, 5624; and, E. M. Ali, Y. Zheng, H. Yu, J. Y. Ying, “Ultrasensitive Pb2+ Detection by Glutathione-Capped nanoparticles”, Anal. Chem. 2007, 79, 9452-9458. Although some of these systems show great sensitivity for metal ions with detection limits as low as 10−11 M, they are limited to laboratory use due to the toxicity of the nanoparticles themselves.
The present invention provides a simple, easily scaled process for producing fluorescent nanoparticles, including quantum dot nanoparticles that are relatively non-toxic and environmentally stable in both air and water. These nanoparticles are made from less toxic metals including but not limited to zinc, silver, indium, and copper. Interaction between the nanoparticles and a target analyte (particularly metal ions, both cations and anions) are used for sensing applications. In contrast to current nanoparticle systems, the nanoparticle systems of the present invention lend themselves well to testing in a non-laboratory environment as they are relatively non-toxic and environmentally stable in both air and water. The current invention encompasses several methods of detection including: shifting of the wavelength of fluorescence, an enhancement of fluorescence, or a quenching of fluorescence when a specific target element or molecule is present.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.