Authentication of quantum dot security inks

1. A method for verifying the authenticity of an article which bears a security mark, comprising:

irradiating the security mark with a time-varying light source;

ascertaining at least one portion of the emissions spectrum of the irradiated security mark with at least one photodetector;

determining the photoluminescence lifetime of said security mark by monitoring the time or frequency response of said photodetector; and

verifying the authenticity of the article only if the security mark exhibits a photoluminescence which has a lifetime that falls within the range of appropriate values for each portion of the photoluminescence spectrum for which the photoluminescence lifetime of said security mark was ascertained;

wherein said security mark emits an emission spectrum having first and second distinct regions which are characterized by first and second distinct lifetimes;

wherein the authenticity of the article is verified only if the security mark exhibits a photoluminescence which has first and second lifetimes that fall within first and second ranges of appropriate values for the first and second regions, respectively; and

wherein the security mark comprises a first set of quantum dots which exhibit photoluminescence over the first region, and a second set of quantum dots which exhibit photoluminescence over the second region;

wherein the security mark is characterized by light emission having wavelengths in the near-IR region of the spectrum, wherein the security mark is characterized by a lifetime that varies by at least 5% across said emission spectrum, and wherein the security mark is characterized by a lifetime that varies by at least 50 ns across said emission spectrum.

2. The method of claim 1 , wherein the first and second sets of quantum dots are chemically or dimensionally distinct.

3. The method of claim 1 , wherein the security mark comprises quantum dots with compositions selected from the group consisting of CuInS 2 , CuInSe 2 , AgInSe 2 , AgInSe 2 , ZnSe and ZnS.

4. The method of claim 1 , wherein the security mark is characterized by a lifetime of more than 40 nanoseconds but less than 1 millisecond.

5. The method of claim 1 , wherein the security mark is characterized by a quantum yield that is greater than 50%.

6. The method of claim 1 , further comprising a handheld device that includes said light source and said at least one photodetector.

7. The method of claim 1 , further comprising at least one spectrum-selecting component through which said photoluminescence must pass before reaching said at least one photodetector.

8. The method of claim 1 , further comprising:

determining said photoluminescence lifetime based on the time-varying average light intensity measured by said at least one photodetector.

9. The method of claim 1 , further comprising determining said photoluminescence lifetime based on the phase difference between said light source and said photoluminescence using a phase analyzer.

10. The method of claim 1 , wherein the security mark comprises quantum dots having an inner core and an outer shell.

11. The method of claim 7 , wherein said at least one spectrum selecting optical component is selected from the group consisting of light filters, quantum dot films and colored glasses.

12. The method of claim 1 , wherein irradiating the security mark with a time-varying light source includes irradiating the security mark at a plurality of frequencies.

13. The method of claim 12 , wherein at least one portion of said photoluminescence spectrum emitted by the irradiated security mark includes first and second distinct portions of said photoluminescence spectrum, and further comprising:

using the average light intensity collected from said at least one photodetector when said security mark is irradiated with said plurality of frequencies to compute upper and lower bounds for the photoluminescence lifetime of each portion of the photoluminescence spectrum for which the photoluminescence lifetime of said security mark was ascertained.