Stabilized semiconductor nanocrystals Number:7,160,613 from the United States Patent and Trademark Office (PTO) owispatent A semiconductor nanocrystal associated with a polydentate ligand. The polydentate ligand stabilizes the nanocrystal.<H semiconductor, nanocrystals, Stabilized, semi, 7160613.html, Patent, pto, 7160613

Title: Stabilized semiconductor nanocrystals

Abstract: A semiconductor nanocrystal associated with a polydentate ligand. The polydentate ligand stabilizes the nanocrystal.

Patent Number: 7,160,613 Issued on 01/09/2007 to Bawendi, et al.

Inventors: Bawendi; Moungi G. (Boston, MA), Kim; Sungjee (Cambridge, MA), Stott; Nathan E. (Cambridge, MA)
Assignee: Massachusetts Institute of Technology (Cambridge, MA)

Appl. No.: 10/641,292

Filed: August 15, 2003

Current U.S. Class: 428/403 ; 252/301.4R; 252/301.4S; 252/301.6R; 252/301.6S; 428/407; 428/690

Current International Class: B32B 5/16 (20060101)

Field of Search: 428/403,407,690 252/301.4R,301.4S,301.6R,301.6S

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5525377	June 1996	Gallagher et al.
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5985173	November 1999	Gray et al.
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6054495	April 2000	Markowitz et al.
6103868	August 2000	Heath et al.
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6139585	October 2000	Li
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Foreign Patent Documents


WO 97/10175	Mar., 1997	WO
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Primary Examiner: Kiliman; Leszek B.
Attorney, Agent or Firm: Steptoe & Johnson LLP

Government Interests

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government may have certain rights in this invention pursuant to Contract No. N00014-01-1-0787 awarded by the Office of Naval Research.

Parent Case Text

CLAIM OF PRIORITY

This application claims priority to provisional U.S. Patent Application Ser. No. 60/403,367, filed on Aug. 15, 2002, the entire contents of which are hereby incorporated by reference.

Claims

What is claimed is:

1. A polydentate ligand of formula: ##STR00015## wherein n is 1, 2, 3, 4 or 5, m is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, each k is 1, 2, 3, or 4, each X independently is a donor group selected from the group consisting of N, P, P.dbd.O, and N.dbd.O, each Y is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted aryl, or substituted or unsubstituted aryloxy, and L is a linking group optionally terminated by O and includes at least one carbonate, carbamate, amide, ester or ether linkage.

2. A polydentate ligand of formula: ##STR00016## wherein n is 1, 2 or 3, m is 1, 2, 3,4, or 5, each k is 1 or 2, each X independently is a donor group selected from the group consisting of N, P, P.dbd.O, and N.dbd.O, each Y is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted aryl, or substituted or unsubstituted aryloxy, and L is a linking group optionally terminated by O and includes at least one carbonate, carbamate, amide, ester or ether linkage.

3. A distribution of polydentate ligands of formula: ##STR00017## wherein n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each X independently is a donor group selected from the group consisting of N, P, P.dbd.O, and N.dbd.O, each Y is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted aryl, or substituted or unsubstituted aryloxy, L is a linking group optionally terminated by O and includes at least one carbonate, carbamate, amide, ester or ether linkage, and the distribution includes at least two members with different values of m.

4. A polydentate ligand of formula: ##STR00018## wherein p is 1 or 2, each m is 1, 2, 3, 4, or 5, each k is 1 or 2, each j is 0 or 1, each p is 0 or 1, q is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, each X independently is a donor group selected from the group consisting of N, P, P.dbd.O, and N.dbd.O, each Y is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted aryl, or substituted or unsubstituted aryloxy, and L is a linking group optionally terminated by O and includes at least one carbonate, carbamate, amide, ester or ether linkage.

5. A polydentate ligand of formula: ##STR00019## wherein n is 1, 2 or 3, m is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, each k is 1 or 2, each x independently is 0 or 1, each of Z.sup.1 and Z.sup.2, independently, is an ether, amide, ester, carbamate or carbonate linkage, each R.sup.1 and R.sup.2, independently, is an alkylene optionally interrupted by S, O, NH, N-lower alkyl, arylene, heteroarylene, or aralkylene and optionally terminated by S, O, NH, N-lower alkyl, arylene, heteroarylene, or aralkylene, and each R is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, or substituted or unsubstituted aryl.

6. A polydentate ligand of formula: ##STR00020## wherein n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each x independently is 0 or 1, Z is an ether, carbamate, amide, ester or carbonate linkage, each R.sup.1 and each R.sup.2, independently, is an alkylene optionally interrupted by S, O, NH, N-lower alkyl, arylene, heteroarylene, or aralkylene, and optionally terminated by S, O, NH, N-lower alkyl, arylene, heteroarylene, or aralkylene, and each R is substituted or unsubstituted alkyl, or substituted or unsubstituted aryl, and each R is bonded to R.sup.1 via an ether, ester, amide, carbamate or carbonate linkage.

7. A polydentate ligand of formula: ##STR00021## wherein n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each x independently is 0 or 1, and each R is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, or substituted or unsubstituted aryl.

8. The ligand of claim 7, wherein at least one x is 0.

9. The ligand of claim 7, wherein each x is 0.

10. The ligand of claim 7, wherein each R is unsubstituted alkyl.

11. The ligand of claim 7, wherein each R includes a carboxylic acid group.

12. The ligand of claim 7, wherein each R includes an acrylate group.

13. A method of making a polydentate ligand comprising contacting a monomeric, polyfunctional phosphine with a polyfunctional oligomerization reagent to form an oligomeric phosphine.

14. The method of claim 13, wherein the monomeric, polyfunctional phosphine is trishydroxypropylphosphine.

15. The method of claim 14, wherein the polyfunctional oligomerization reagent is a diisocyanate.

16. The method of claim 13, further comprising contacting the oligomeric phosphine with an isocyanate of formula: R'--L--NCO, wherein L is C.sub.2 C.sub.24 alkylene, and R' has the formula: ##STR00022## R' has the formula: ##STR00023## or R' is hydrogen, wherein R.sup.a is hydrogen or C.sub.1 C.sub.4 alkyl.

17. A method of making a nanocrystal-biomolecule conjugate comprising contacting a nanocrystal including a polydentate ligand including a reactive group with a biomolecule.

18. The method of claim 17, wherein the biomolecule is a polypeptide.

19. The method of claim 17, further comprising contacting the nanocrystal and the biomolecule with a cross-linking agent.

20. The method of claim 19, wherein the reactive group is a carboxylic acid.

21. The method of claim 20, wherein the biomolecule includes an amino group and the cross-linking agent is a carbodiimide.

Description

TECHNICAL FIELD

The invention relates to stabilized semiconductor nanocrystals.

BACKGROUND

Semiconductor nanocrystals have been a subject of great interest, promising extensive applications including display devices, information storage, biological tagging materials, photovoltaics, sensors and catalysts. Nanocrystals having small diameters can have properties intermediate between molecular and bulk forms of matter. For example, nanocrystals based on semiconductor materials having small diameters can exhibit quantum confinement of both the electron and hole in all three dimensions, which leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of nanocrystals shift to the blue (i.e., to higher energies) as the size of the crystallites decreases. Semiconductor nanocrystals can have a narrow fluorescence band whose emission wavelength is tunable with the size and material of the nanocrystals.

Nanocrystals consist of an inorganic nanoparticle that is surrounded by a layer of organic ligands. This organic ligand shell is critical to the nanocrystals for processing, binding to specific other moieties, and incorporation into various substrates. Fluorescent nanocrystals are most stable and robust when there is an excess amount of passivating ligands in solution. Monodentate alkyl phosphines and alkyl phosphine oxides passivate nanocrystals efficiently. Note that the term phosphine will refer to both phosphines and phosphine oxides below. Nanocrystals can be stored in their growth solution, which contains a large excess of ligands such as alkyl phosphines and alkyl phosphine oxides, for long periods without noticeable degradation. For most applications, nanocrystals must be processed outside of their growth solution and transferred into various chemical environments. However, nanocrystals often lose their high fluorescence or become irreversibly aggregated when removed from their growth solution.

SUMMARY

In general, a semiconductor nanocrystal having a polydentate ligand on the surface of the nanocrystal can be stabilized in comparison to a nanocrystal having a monodentate ligand on the surface of the nanocrystal. Monodentate ligands can readily exchange and diminish or quench emission from the nanocrystal as a result of the exchange. When nanocrystals with conventional monodentate ligands are diluted or embedded in a non-passivating environment (i.e. one where no excess ligands are present), the nanocrystals tend to lose their high luminescence and their initial chemical inertness, as manifested by, for example, an abrupt decay of luminescence, aggregation, and/or phase separation. The polydentate ligand can be a polyphosphine, a polyphosphine oxide, a polyphosphinic acid, or a polyphosphonic acid, or a salt thereof.

Advantageously, polydentate ligands, particularly oligomerized polydentate ligands such as polydentate oligomerized phosphine ligands, bind more strongly to the surface of the nanocrystal than monodentate ligands. Polydentate ligands thus stabilize the nanocrystal, which can preserve the high luminescence of as-grown nanocrystals. Polydentate phosphines can be more securely anchored onto the nanocrystal surface than bidentate thiols. In a tagging application, for example, they can ensure more secure chemical attachments of tags to their targets. In addition, because of the affinity of the polydentate ligands for the nanocrystal, minimal amounts of oligomeric phosphines can be used to passivate nanocrystals since the higher affinity and compatibility ensures a high local concentration of the ligand around the nanocrystal surface. The polydentate ligand provides a local environment that is very similar to its growth solution because the growth solution is the medium where the nanocrystal is most stable. The polydentate phosphine provides a high density phosphine ligand layer on the nanocrystal surface. Also advantageously, the outer portion of the polydentate ligand, can be chosen to be compatible with the bulk environment surrounding the nanocrystal, such as an organic solvent, aqueous media, or polymer matrix. The polydentate ligands are chemically flexible so that they can be easily functionalized to be compatible with a variety of chemical environments. For example, the polydentate ligands can be functionalized to be hydrophobic, hydrophilic, or polymerizable.

In one aspect, a semiconductor nanocrystal includes a semiconductor nanocrystal and an outer layer comprising a polydentate ligand bonded to the nanocrystal by three or more donor groups, each donor group independently selected from the group consisting of P, N, P.dbd.O, and N.dbd.O. The polydentate ligand can be a member of a distribution of oligomers. In another aspect, a semiconductor nanocrystal includes a semiconductor nanocrystal, and an outer layer including a plurality of polydentate ligands, each polydentate ligand bound to the nanocrystal by three or more donor groups, each donor group independently selected from the group consisting of P, N, P.dbd.O, and N.dbd.O, the plurality of polydentate ligands being a distribution of oligomers.

In another aspect, a semiconductor nanocrystal includes a semiconductor nanocrystal and an outer layer including a polydentate ligand bound to the nanocrystal by three or more donor groups, each donor group independently selected from the group consisting of P, N, P.dbd.O, and N.dbd.O, wherein the luminescence of the nanocrystal decreases by no more than 50% after incubating for 24 hours in fetal bovine serum maintained at 37.degree. C.

In another aspect, a method of making a stabilized nanocrystal includes contacting a nanocrystal with a polydentate ligand having three or more donor groups, each donor group independently selected from the group consisting of P, N, P.dbd.O, and N.dbd.O, to form the stabilized nanocrystal. Stabilizing the nanocrystals can include cross-linking the polydentate ligand. The polydentate ligand can include a carboxylic acid, and cross-linking can include contacting the polydentate ligand with a diamine and a coupling agent. The polydentate ligand can include an acrylate group, and cross-linking can include contacting the polydentate ligand with a radical initiator.

In another aspect, a method of making a polydentate ligand includes contacting a monomeric, polyfunctional phosphine with a polyfunctional oligomerization reagent to form an oligomeric phosphine. The monomeric, polyfunctional phosphine can be trishydroxypropylphosphine. The polyfunctional oligomerization reagent can be a diisocyanate. The oligomeric phosphine can be contacted with an isocyanate of formula R'--L--NCO, wherein L is C.sub.2 C.sub.24 alkylene, and R' has the formula

##STR00001## R' has the formula

##STR00002## or R' is hydrogen, wherein R.sup.a is hydrogen or C.sub.1 C.sub.4 alkyl.

In yet another aspect, a method of making a nanocrystal-biomolecule conjugate includes contacting a nanocrystal including a polydentate ligand including a reactive group with a biomolecule. The biomolecule can be a polypeptide. The nanocrystal and the biomolecule can be contacted with a cross-linking agent. The reactive group can be a carboxylic acid. The biomolecule can include an amino group and the cross-linking agent can be a carbodiimide.

The first semiconductor material can be a Group II VI compound, a Group II V compound, a Group III VI compound, a Group III V compound, a Group IV VI compound, a Group I III VI compound, a Group II IV VI compound, or a Group II IV V compound, such as, for example, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TiAs, TlSb, PbS, PbSe, PbTe, or mixtures thereof. Each first semiconductor material can be overcoated with a second semiconductor material, such as ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, TlSb, PbS, PbSe, PbTe, or mixtures thereof. The nanocrystal can be a member of a monodisperse distribution of sizes of nanocrystals. The first semiconductor material can have a smaller band gap than the second semiconductor material.

Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram depicting representative chemical structures of oligomeric phosphines.

FIG. 2 is a graph depicting the mass spectrum of oligomeric phosphine.

FIG. 3 is a set of graphs depicting quantum yield changes over time of identical CdSe/ZnS nanocrystals passivated by different ligands.

DETAILED DESCRIPTION

Nanocrystal cores can be prepared by the pyrolysis of organometallic precursors in hot coordinating agents. See, for example, Murray, C. B., et al., J. Am. Chem. Soc. 1993, 115, 8706, and Mikulec, F., Ph.D. Thesis, MIT, Cambridge, 1999, each of which is incorporated by reference in its entirety. Growth of shell layers on the bare nanocrystal cores can be carried out by simple modifications of conventional overcoating procedures. See, for example, Peng, X., et al., J. Am. Chem. Soc. 1997, 119, 7019, Dabbousi, B. O., et al., J. Phys. Chem. B 1997, 101, 9463, and Cao, Y. W. and Banin, U. Angew. Chem. Int. Edit. 1999, 38, 3692, each of which is incorporated by reference in its entirety.

A coordinating agent can help control the growth of the nanocrystal. The coordinating agent is a compound having a donor lone pair that, for example, has a lone electron pair available to coordinate to a surface of the growing nanocrystal. The coordinating agent can be a solvent. A coordinating agent can stabilize the growing nanocrystal. Typical coordinating agents include alkyl phosphines, alkyl phosphine oxides, alkyl phosphonic acids, or alkyl phosphinic acids, however, other coordinating agents, such as pyridines, furans, and amines may also be suitable for the nanocrystal production. Examples of suitable coordinating agents include pyridine, tri-n-octyl phosphine (TOP) and tri-n-octyl phosphine oxide (TOPO). Technical grade TOPO can be used.

The outer surface of the nanocrystal can include a layer of compounds derived from the coordinating agent used during the growth process. The surface can be modified by repeated exposure to an excess of a competing coordinating group to form an overlayer. For example, a dispersion of nanocrystals capped with the coordinating agent used during growth can be treated with a coordinating organic compound, such as pyridine, to produce crystallites which disperse readily in pyridine, methanol, and aromatics but no longer disperse in aliphatic solvents. Such a surface exchange process can be carried out with any compound capable of coordinating to or bonding with the outer surface of the nanocrystal, including, for example, phosphines, thiols, amines and phosphates. The nanocrystal can be exposed to short chain polymers which exhibit an affinity for the surface and which terminate in a moiety having an affinity for a suspension or dispersion medium. Such affinity improves the stability of the suspension and discourages flocculation of the nanocrystal.

Monodentate alkyl phosphines and alkyl phosphine oxides passivate nanocrystals efficiently. Note that the term phosphine will refer to both phosphines and phosphine oxides below. Other conventional ligands such as thiols or phosphonic acids can be less effective than monodentate phosphines for maintaining the initial high nanocrystal luminescence over long periods. For example, the photoluminescence of nanocrystals consistently diminishes or quenches after ligand exchanges with thiols or phosphonic acid.

An excess of free monodentate phosphine ligands can maintain high nanocrystal luminescence. An excess of free phosphine ligands can favor a nanocrystal surface that is densely covered by the passivating ligands. When nanocrystals with conventional monodentate ligands are diluted or embedded in a non-passivating environment (i.e. an environment where excess ligands are not present), however, the nanocrystals can lose their high luminescence and chemical inertness. In such an environment, typical effects can include an abrupt loss of luminescence, aggregation, and/or phase separation.

In order to overcome the limitations of monodentate ligands, polydentate ligands, such as a distribution of oligomeric polydentate phosphine ligands, can be used. Polydentate ligands show a high affinity for the nanocrystal surface. In other words, a polydentate ligand can have a larger equilibrium constant for binding to a nanocrystal than a chemically similar monodentate ligand. Oligomeric phosphines have more than one binding site to the nanocrystal surface, which contributes to their high affinity for the nanocrystal surface. Oligomeric phosphines can be preferred to bidentate thiols as nanocrystal ligands because oligomeric phosphines can preserve the high luminescence of as-grown nanocrystals. Moreover, polydentate phosphines can be more securely anchored onto (i.e., have a higher affinity for) the nanocrystal surface than bidentate thiols. In a tagging application, for example, the polydentate ligand can ensure a more secure chemical attachment of a tag to its target that a monodentate ligand. Minimal amounts of oligomeric phosphines can be used to passivate nanocrystals. Unlike monodentate ligands, an excess of oligomeric phosphines is not necessary to maintain the high luminescence of nanocrystals. Oligomeric phosphines can provide the nanocrystal surface with a local environment that is very similar to its growth solution, where the nanocrystal is most stable. Polydentate phosphines can form a high-density phosphine ligand layer on the nanocrystal surface. To prevent aggregation or phase separation of nanocrystals, the outermost surface of nanocrystal must be compatible to the bulk environment. The ligands can be easily functionalized to be compatible with a variety of chemical environments. For instance, they can be functionalized to be hydrophobic, hydrophilic, or polymerizable.

The polydentate ligand can be an oligomer, or a distribution of oligomers. The polydentate ligand can have the formula:

##STR00003## where n is 1, 2, 3, 4 or 5, m is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, each k is 1, 2, 3, or 4, each X is N P, P.dbd.O or N.dbd.O, each Y is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted aryl, or substituted or unsubstituted aryloxy, and L is a linking group optionally terminated by O and includes at least one carbonate, carbamate, amide, ester or ether linkage.

The polydentate ligand can be of the formula:

##STR00004## where n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each X is N, P, P.dbd.O or N.dbd.O, each Y is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted aryl, or substituted or unsubstituted aryloxy, and L is a linking group optionally terminated by O and includes at least one carbonate, carbamate, amide, ester or ether linkage.

The polydentate ligand can have the formula:

##STR00005## where p is 1 or 2, each m is 1, 2, 3, 4, or 5, each k is or 2, each j is 0 or 1, each p is 0 or 1, q is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, each X is N, P, P.dbd.O or N.dbd.O, each Y is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted aryl, or substituted or unsubstituted aryloxy, and L is a linking group optionally terminated by O and includes at least one carbonate, carbamate, amide, ester or ether linkage.

In certain circumstances, X is P or P.dbd.O, and L includes at least on carbamate linkage. In certain circumstances, each Y can be unsubstituted alkyl, each Y can include a carboxylic acid, or each Y can include an acrylate group.

The polydentate ligand can have the formula:

##STR00006## where n is 1, 2 or 3, m is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, each k is 1 or 2, each x independently is 0 or 1, each of Z.sup.1 and Z.sup.2, independently, is an ether, amide, ester, carbamate or carbonate linkage, each R.sup.1 and R.sup.2, independently, is an alkylene optionally interrupted by S, O, NH, N-lower alkyl, arylene, heteroarylene, or aralkylene and optionally terminated by S, O, NH, N-lower alkyl, arylene, heteroarylene, or aralkylene, and each R is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, or substituted or unsubstituted aryl. In certain embodiments, Z.sup.1 and Z.sup.2 are each a carbamate linkage. In certain circumstances, R.sup.1 and R.sup.2 are each an alkylene.

The polydentate ligand can have the formula:

##STR00007## where n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each x independently is 0 or 1, Z is an ether, carbamate, amide, ester or carbonate linkage, each R.sup.1 and each R.sup.2, independently, is an alkylene optionally interrupted by S, O, NH, N-lower alkyl, arylene, heteroarylene, or aralkylene, and optionally terminated by S, O, NH, N-lower alkyl, arylene, heteroarylene, or aralkylene, and each R is substituted or unsubstituted alkyl, or substituted or unsubstituted aryl, and each R is bonded to R.sup.1 via an ether, ester, amide, carbamate or carbonate linkage.

The polydentate ligand can have the formula:

##STR00008## where n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each x independently is 0 or 1, and each R is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, or substituted or unsubstituted aryl. R can have the formula:

##STR00009##

The polydentate ligand can be cross-linked once bound to a nanocrystal. The cross-linked polydentate ligand can have the formula:

##STR00010## where each n independently is 1, 2 or 3, each m independently is 1, 2, 3, 4, or 5, each k is 1 or 2, each X is N, P, P.dbd.O or N.dbd.O, each Y is substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted aryl, or substituted or unsubstituted aryloxy, L is a linking group optinally terminated by O and includes at least one carbonate, carbamate, amide, ester or ether linkage, L' is a bond or a cross-linking group, and Y'--L'--Y' is derived from cross-linking of Y.

The cross-linked polydentate ligand can have the formula:

##STR00011## where n is 1, 2 or 3, m is 1, 2, 3, 4, or 5, each k is 1 or 2, each x is 0 or 1, and each R is substituted or unsubstituted alkyl, or substituted or unsubstituted aryl, L' is a bond or a cross-linking group, and R'--L'--R' is derived from cross-linking of R. When each R includes a carboxylic acid, the polydentate ligand can be cross-linked with, for example, a diamine, and R'--L'--R' can include the fragment:

##STR00012##

where A is alkylene or arylene. When each R includes an acrylate group, the polydentate ligand can be cross-linked by radical polymerization of the acrylate groups, and R'--L'--R' can include the fragment:

##STR00013## where A' is H or C.sub.1 C.sub.4 alkyl.

FIG. 1 shows chemical structures of representative oligomeric phosphines with functionalized branches. The exemplary functional groups shown are alkyl, methacrylate, and carboxylic acid. Many other functional groups can be introduced with minor modifications to the synthesis. This flexibility can allow homogeneous incorporation of nanocrystals in any desired medium.

The oligomeric ligands can create a trilayer around the nanocrystal: a phosphine layer, a hydrophobic linking layer, and a functionalized layer. The phosphine layer can passivate the nanocrystal surface, the hydrophobic layer can protect it, while the functionalized layer can deliver desirable chemical properties including solubility, miscibility, the ability to copolymerize with other matrices, further cross-linking on the surface of the nanocrystals, and other derivatizations such as conjugation to biomolecules.

The synthesis of oligomeric phosphines (such as those shown in FIG. 1) and methods for ligand exchange on nanocrystal surfaces are described below. The synthesis is flexible and can be easily modified. In general, a monomeric phosphine is oligomerized, and the resulting oligomeric phosphine is functionalized. A specific example is shown in Scheme 1, which can be easily generalized and modified to synthesize the polydentate ligands described here. As shown in Scheme 1, a monomeric phosphine such as trishydroxypropylphosphine (THPP) can be oligomerized by reaction with a multifunctional linker such as diisocyanatohexane (DIH). Though Scheme 1 shows a linear oligomer, branched oligomers are possible. The linker can be a bifunctional, trifunctional or higher functional linker. The distribution of oligomers can be controlled by adjusting the stoichiometry of the monomeric unit and linker. In certain circumstances, the distribution of oligomers includes primarily oligomers with n=1, 2, 3, or 4 according to Scheme 1. Many other linkers can also be used. Various alkyldiisocyanates with different length alkyl chains and aryldiisocyanates are commercially available (for example, from Sigma-Aldrich) and can act as varying length spacers between phosphine groups within the oligomers.

The oligomeric phosphine can be functionalized, for example by reaction with a second isocyanate including a group that bestows a desired property on the functionalized oligomeric phosphine. The second isocyanate is represented in Scheme 1 as R--NCO. For example, if the desired property is hydrophobicity, the second isocyanate can include a hydrophobic group such as an alkyl chain, as in octyl isocyanate or hexadecyl isocyanate. Other examples of properties that can be introduced include hydrophilicity (e.g. from a hydrophilic group such as a carboxylic acid) and ability to polymerize (e.g. from a polymerizable group such as an acrylate or methacrylate). See FIG. 1. In some circumstances, the ligand can be exposed to oxygen (for example, air) to oxidize the donor atoms (i.e. P or N).

Chemical functionality can be introduced to the small oligomeric phosphine by further reactions with any molecule or a combination of molecules. The functionality can be introduced, for example, by reaction of an oligomeric phosphine having unreacted hydroxyl groups with a molecule having a desired functional group and an isocyanate group. See Scheme 1. For example, octylisocyanate or hexadecylisocyanate can be used to introduce a hydrophobic alkyl chain, and a polymerizable methacrylate group can be introduced by reaction with 2-isocyanatoethylmethacrylate. In some cases, conventional protection and deprotection procedures on the desired functional group may be necessary to facilitate synthesis. An oligomeric phosphine bearing carboxylic acid groups (FIG. 1) can be prepared by hydrolysis of an ester derivatized oligomeric phosphine. The ester derivatized oligomeric phosphine can prepared from the reaction between the oligomeric phosphine and methyl-5-isocyanatopentanoate. Advantageously, the ester can be selectively hydrolyzed under basic hydrolysis conditions while retaining the carbamate linkages.

Carbamate bond formation between a monomeric phosphine, such as THPP, and a diisocyanate such as DIH can be advantageous as an oligomerization reaction. Advantages of this oligomerization reaction include a reaction to completeness under mild conditions at room temperature. The monomeric phosphine, in addition to serving as a reactant, can catalyze the carbamate bond formation reaction. Tin compounds such as dibutyltin dilaurate can be added to further catalyze the reaction. See, for example, Ulrich, H., Chemistry and technology of isocyanates 1996, Chichester, N.Y. , J. Wiley & Sons, which is incorporated by reference in its entirety. Another advantage is the small extent of side reactions, such that purification can be unnecessary. An additional advantage is that the carbamate bond can be stable enough for most purposes such as fluorescence in situ hybridization procedures. See, for example, Pathak, S., et al., 2001 J. Am. Chem. Soc. 123, 4103, and Palm, V. A., Tables of rate and equilibrium constants of heterolytic organic reactions V.1 1975 Laboratory of chemical kinetics and catalysis at Tartu State University, Moscow, each of which is incorporated by reference in its entirety.

In one example of a polydentate ligand, FIG. 2 shows a mass spectrum of an unfunctionalized oligomeric phosphine, and reveals a narrow distribution of oligomers. Labels a), b), c) and d) indicate peaks that correspond to the oligomeric phosphine depicted in Scheme 1, with n=1, n=2, n=3, and n=4, respectively. The mass spectrum was recorded with a Bruker Daltonics APEX3 with an electrospray ionization source. Peaks from multiple charges were deconvoluted to singly charged mass numbers to demonstrate the distribution of oligomers.

##STR00014##

Ligand exchanges (e.g. substitution of an oligomeric phosphine for a monodentate phosphine) can be carried out by one-phase or two-phase methods. Prior to ligand exchange, nanocrystals can be precipitated from their growth solutions by addition of methanol. The supernatant solution, which includes excess coordinating agent (e.g., trioctylphosphine), can be discarded. The precipitated nanocrystals can be redispersed in hexanes. Precipitation and redispersion can be repeated until essentially all the excess coordinating agent has been separated from the nanocrystals. A one-phase process can be used when both the nanocrystals and the ligands to be introduced are soluble in the same solvent. A solution with an excess of new ligands can be mixed with the nanocrystals. The mixture can be stirred at an elevated temperature until ligand exchange is complete. The one-phase method can be used, for example, to exchange octyl-modified oligomeric phosphines or methacrylate-modified oligomeric phosphines, which are both soluble in solvents that are compatible with the nanocrystals, such as hexanes. A two-phase ligand exchange process can be preferable when the nanocrystals and the new ligands do not have a common solvent. Nanocrystals can dissolved in an organic solvent such as dichloromethane, and the new ligand can be dissolved in an aqueous solution. The nanocrystals can be transferred from the organic phase to the aqueous phase by, for example, sonication. The transfer can be monitored through absorption and emission spectroscopy. A carboxylic acid-modified oligomeric phosphine can be introduced to nanocrystals via this method. A similar two-phase ligand exchange process has been reported earlier. See, for example, Wang, Y. A., et al., 2002 J. Am. Chem. Soc 124, 2293, incorporated by reference in its entirety.

FIG. 3 shows a comparison of nanocrystal stability in the presence of oligomeric phosphine ligands or monomeric ligands. The comparison was made in organic solvent and in aqueous solution. Equimolar bindin