Luminescent nanomaterials

Luminescent nanomaterials (Fig. 1) are, in principle, relevant to all the well-known applications of classical bulk-phosphors (i.e., fluorescent lamps, light-emitting diodes, emissive displays, tomographs, X-ray detectors).¹ Since synthesis and materials are typically more elaborate and costly on the nanoregime, however, luminescent nanomaterials only become relevant if an additional decisive advantage and function is in reach. This comprises luminescent, but optically transparent thin-films of nanoparticles on/in glass, paper or polymers for issues of marking (e.g. advertising, emergency lighting) or security (e.g. banknotes, negotiable instruments, identification documents).^2,3 While any on/off-switching or color shift of an emission is easy to detect – most straightforward by the naked eye – luminescent thin-films and porous matrices can be ideal for sensor effects, too. The most ambitious and fascinating application of luminescent nanomaterials is probably related to medicine and molecular biology.^3–5 Here, luminescent nanomaterials are promising tags for optical imaging and fluorescent labeling to allow for novel techniques of non-invasive diagnosis and in vivo observation of complex vital functions. This can range from in vivo whole-body diagnosis to in vitro examination of individual organs or cells.

Fig. 1 Scheme illustrating the range of luminescent nanomaterials.

For practical use of luminescent nanomaterials, various aspects have to be considered. Most often the emission intensity – or more accurately the contrast between a luminescent material and a (non-luminescent) background/substrate – plays a major role. Here, the quantum yield (viz. the ratio of emitted and absorbed photons) as well as the number of luminescent centers per volume are highly relevant.^1,3 Only if both characteristics are high, is a considerable emission observed. Furthermore, absorption/emission characteristics, luminescence energy-transfer, decay time, chemical/physical/colloidal stability, materials expenses, reproducibility of manufacture and materials toxicity/biocompatibility have to be taken into account. The weighting of these aspects depends on each specific material and application.

With regard to its composition, luminescent nanomaterials can be generally assigned to four classes (Fig. 1): (1) semiconductor-type quantum dots (Q-dots) and carbon dots (C-dots); (2) metal nanoclusters; (3) metal-doped nanomaterials; (4) organic–inorganic composites and hybrids. Semiconductor-type quantum dots such as CdSe or CdTe may be considered the most widely applied luminescent nanomaterials so far.^6,7 The major breakthroughs here are related to precise control of particle sizes, the realization of advanced core–shell structures and surface-cappings as well as avoiding the detrimental “blinking” (i.e. the non-continuous emission intensity characteristics). By now, a huge variety of II–VI, III–V and IV–IV Q-dots can be obtained whose emission covers the complete spectral range from UV to IR, with quite often excellent quantum yields of 60–90%.⁶ Due to the underlying quantum-size effect, different emission colors can be obtained here with just a single material by adapting the particle size. However, Q-dots often suffer from the complexity of synthesis and composition that is accompanied by low material quantities and high material expenses. And finally, the inherent toxicity and the sensitivity of Q-dots to hydrolysis and oxidation can restrict its use. To this concern, the up-coming C-dots as well as Si or ZnO nanoparticles can serve as useful alternatives.^3,8 Moreover, the surface-plasmon resonance can be used to trigger visible emission of metal nanoclusters. This effect is however more or less restricted to nanostructured gold and silver.⁹

While the luminescence of Q-dots, C-dots and metal nanoclusters is related to quantum-confinement effects, a precise adjustment of particle size and surface-capping is prerequisite to an efficient emission with a defined color. In contrast, metal-doped nanomaterials as well as organic–inorganic composites and hybrids involve discrete luminescent centers (i.e. ions, complex ions, molecules).^1,3 The luminescence is therefore on the one hand independent from the particle size, which often facilitates the materials’ synthesis, but which on the other hand restricts the luminescence of a specific material to a single emission color. Well-known examples of metal-doped nanomaterials include, for instance, NaYF₄:Er,Yb, LaPO₄:Ce,Tb, ZnS:Mn or YVO₄:Eu. In view of a strong emission, similar strategies as for the Q-dots are followed, including high purity and crystallinity of the host lattice as well as shielding the luminescent core by suitable core–shell structures.³

Organic–inorganic nanocomposites and hybrids recently came up as a new class of luminescent nanomaterials. Nanocomposites are typically composed of a luminescent organic dye (e.g. phenoxazine, nile red, rhodamine, indocyanine green, fluorescein) that is statistically attached to or incorporated in an inorganic matrix.^10,11 The respective inorganic matrix most often consists of silica or calcium phosphate. Surface-attached dyes, however, hold the risk of abrasive debonding. Encapsulation of dyes inside a matrix is normally performed via microemulsion techniques, which significantly limits the available material quantities. In contrast to composites, organic–inorganic hybrids contain the luminescent dye as an intrinsic part of the solid lattice in molar quantities and therefore guarantees the intense spot-light emission of each single nanoparticle. As an example, the luminescent hybrid ZrO(FMN) (FMN: flavin mononucleotide) is formally composed of the inorganic cation [ZrO]²⁺ and the luminescent dye anion [FMN]²⁻.¹² Nanoscaled luminescent composites and hybrids can be interesting alternatives to the well-established Q-dots and metal-doped nanoparticles since high demands on material crystallinity, advanced core–shell structures and harmful elements can be avoided. The long-term thermal and photochemical stability, on the other hand, is much higher in the case of the purely inorganic luminescent nanomaterials.

This themed issue comprises a wide range of nanomaterials with different composition. All these materials, however, are associated by a specific corporate property: the emission of light. Most of the contributions take advantage of the up-coming alternatives to the still most widely applied Q-dots. Thus, Tan et al. (DOI: 10.1039/C0NR01014F) review the recent progress in the development of single-wall carbon nanotubes for biosensing applications. Specifically functionalized C-dots with a quantum yield as high a 78% are presented in a paper by Sun et al. (DOI: 10.1039/C0NR00962H). C-Dots and their surface functionalization are also addressed by Chen et al. (DOI: 10.1039/C0NR00953A) with ferrocene as a redox-active capping whose interaction with the carbon core is investigated by spectroscopic methods as well as by cyclovoltammetry. The minireview by Ras et al. (DOI: 10.1039/C1NR00006C). and the paper by Nienhaus et al. (DOI: 10.1039/C0NR00947D) involve luminescent silver and gold nanoclusters and their use for optical imaging. Lu et al. (DOI: 10.1039/C0NR00929F), moreover, take advantage of Gd³⁺-modified gold nanorods as multimodal imaging probes including optical imaging, MRI and CT. Rare-earth doped nanoparticles such as NaYbF₄:Tm, Y₃Al₅O₁₂:Ce and YP_xV_1−xO₄:Eu are used by Prasad et al. (DOI: 10.1039/C0NR01018A), Revaux et al. (DOI: 10.1039/C0NR01000F) and Yan et al. (DOI: 10.1039/C0NR01006E). And finally, organic–inorganic composites and hybrids are presented in a minireview by Epple et al. (DOI: 10.1039/C1NR00002K) as well as by papers of Cao et al. (DOI: 10.1039/C0NR00956C), Nienhaus et al. (DOI: 10.1039/C0NR00944J), Adair et al. (DOI: 10.1039/C0NR00995D), Sokolov et al. (DOI: 10.1039/C0NR01015D) and Liu et al. (DOI: 10.1039/C0NR00950D).

As diverse as the materials composition is as wide is the field of application of luminescent nanomaterials. Most of the contributions, in fact, are related to life science and optical imaging. In concrete this includes fluorescent assays for proteins and DNA detection (cf. Tan et al.), cellular imaging (cf. Nienhaus et al., Epple et al., Liu et al. and Cao et al.) and multimodal imaging techniques (cf. Lu et al.). Despite the importance of this challenging field, however, several additional highly relevant aspects are addressed, too. This comprises thin-films (cf. Ras et al.) and sensing effects (cf. Suslick et al. (DOI: 10.1039/C0NR00963F)). Finally, fundamental aspects of nanoscale luminescent materials are still a challenge and an issue for proceeding optimization. Thus, Prasad et al., Yan et al. and Chen et al. report on surface functionalization including core–shell structures as well as on the colloidal stability of nanoscale luminescent materials. Mechanism and intensity of the luminescent processes are addressed by Revaux et al. as well as by Sun et al.

In summary, the collection of papers presented in this themed issue of Nanoscale illustrates the complexity and relevance of luminescent nanomaterials as a highly agile research field. Although many materials are already available, the use of custom-made luminescent nanoparticles as oligofunctional entities still requires versatile studies that address aspects such as: (1) straightforward synthesis; (2) specific surface conditioning; (3) highly intense light emission; (4) highly specific, easy-to-detect luminescence; (5) sufficient biocompatibility. These general preconditions will be especially important in view of a broad application of luminescent nanomaterials in molecular biology and medicine.

References

1. Phosphor Handbook, ed. S. Shionoya and W. M. Yen, CRC Press, Boca Raton, 1999.
2. H. Althues, J. Henle and S. Kaskel, Chem. Soc. Rev., 2007, 36, 1454.
3. H. Goesmann and C. Feldmann, Angew. Chem., Int. Ed., 2010, 49, 1362.
4. J. G. Fujimoto and D. Farkas, Biomedical Optical Imaging, Oxford University Press, Oxford, 2009.
5. K. Riehemann, S. W. Schneider, T. A. Luger, B. Godin, M. Ferrari and H. Fuchs, Angew. Chem., 2009, 121, 886; K. Riehemann, S. W. Schneider, T. A. Luger, B. Godin, M. Ferrari and H. Fuchs, Angew. Chem., Int. Ed., 2009, 48, 872.
6. G. Schmid (Ed.), Nanoparticles, Wiley-VCH, Weinheim, 2004, pp. 4, pp. 50, pp. 305.
7. B. Nitzsche, F. Ruhnow and S. Diez, Nat. Nanotechnol., 2008, 3, 552.
8. S. N. Baker and G. A. Baker, Angew. Chem., Int. Ed., 2010, 49, 6726–6744.
9. G. Hodes, Adv. Mater., 2007, 19, 639.
10. I. Sokolov and S. Naik, Small, 2008, 4, 934.
11. T. T. Morgan, H. S. Muddana, E. I. Altinoglu, S. M. Rouse, T. Tabakivic, T. Tabouillot, T. J. Russin, S. S. Shanmugavelandy, P. J. Butler, P. C. Eklund, J. K. Yun, M. Kester and J. H. Adair, Nano Lett., 2008, 8, 4108.
12. M. Roming, H. Lünsdorf, K. E. J. Dittmar and C. Feldmann, Angew. Chem., Int. Ed., 2010, 49, 632.

---