We demonstrate the conceptual advantage of using metal-organic frameworks (MOFs) for the creation of a polymetallic material that contains several different near-IR-emitting lanthanide cations and operates as a barcode material with unique luminescence properties. By choosing the ratio of lanthanide salts used during the synthesis, we can control the ratio of lanthanide cations present in the resulting material. We have demonstrated that the emission intensity of each of the different lanthanide cations is proportional to its amount in the MOF crystal, resulting in unique spectroscopic barcodes that depend on the lanthanide cation ratios and compositions.

The electronic structure of CdSe semiconductor nanocrystals has been used to sensitize Tb3+ in solution by incorporation of Tb3+ cations into the nanocrystals during synthesis. Doping of luminescent Tb3+ metal ions in semiconductor nanocrystals utilizes the positive attributes of both species' photophysical properties, resulting in a final product with long luminescence lifetimes, sharp emission bands, high absorptivities, and strong resistance to decomposition. This strategy also helps protect the lanthanide cations from nonradiative deactivation from C-H, N-H, and O-H oscillators of solvent molecules or traditional organic lanthanide ligands, leading to long Tb3+ luminescence lifetimes. This new type of nanomaterial synergistically combines the photophysical properties of nanocrystals and Tb3+.

A strategy to sensitize and protect near-infrared (NIR) emitting Nd 3+ and Yb 3+ is presented. Combining protection provided by the inorganic matrix of NaYF 4 nanocrystals and sensitization from tropolonate ligands capped on their surface, the lanthanide cation centered luminescence was observed through the ligand excitation. The extended lanthanide luminescence lifetimes indicate the success of this strategy.Lanthanide-based near-infrared (NIR) emitters have a great potential to serve as bioanalytical reporters for several reasons: (i) NIR photons scatter less than visible photons, improving image resolution. 1 (ii) Biological systems have low native autofluorescence in the NIR energy domain, 2 resulting in higher detection sensitivity due to improved signal-to-noise ratio. (iii) Most luminescent lanthanide complexes are not susceptible to photodecomposition, allowing long or repeated experiments and simplifying sample storage and preparation procedures. Since f−f transitions are Laporte forbidden, 3 free Ln 3+ have low extinction coefficients resulting in low luminescence intensity. Therefore, it is necessary to sensitize these cations through a suitable chromophore ("antenna effect"), 4 an area of research that has been highly active in recent years. 5 However, this approach has intrinsic limitations because lanthanide luminescence is easily quenched through nonradiative routes when the cations are in close proximity to the vibrational overtones of -OH, -NH, and -CH groups present in the sensitizing ligand and/or solvent. 6 This effect is particularly dramatic for NIR emitting Ln 3+ because of relatively small energy gaps between ground and excited electronic states. 6 12 These materials protect lanthanide cations from sources of nonradiative deactivation; however, they have either limited (e.g., LnVO 4 ) or no absorbance in the UV range. Thus, these inorganic materials are not able to sensitize lanthanide luminescence with the efficiency of organic sensitizers.Here we introduce a strategy to overcome the limited lanthanide sensitization by binding organic tropolonate chromophoric groups to the surface of NaYF 4 nanocrystals (NCs), doped with NIR emitting Nd 3+ or Yb 3+ (Scheme 1). Tropolonate (Trop -) was chosen as a capping ligand since it has been previously demonstrated to be a suitable sensitizer for several lanthanide cations emitting in the NIR range when coordinated in KLn(Trop) 4 complexes. 13 These novel systems use the NaYF 4 matrix to protect Ln 3+ from nonradiative deactivations, while a chromophoric coating sensitizes their luminescence.A synthetic method to prepare Nd 3+ or Yb 3+ doped NaYF 4 NCs was developed on the basis of a recently reported synthesis of NaYF 4 NCs (see Supporting Information). 14 The Trop -capped NCs were synthesized using the following procedure. Tropolone was dissolved in methanol, then deprotonated with an equimolar amount of KOH in methanol. Chloroform was added to obtain a 1/1 (v/v) MeOH/CHCl 3 solvent mixture. This solution was added to a purified solution...