1
Efficient energy transfer between nanocrystalline YAG:Ce and TRITC

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

2
Luminescent nanocrystalline (NC) Ce3+-doped yttrium aluminum garnet (Y3Al5O12:Ce–YAG:Ce) is prepared by a combustion synthesis technique that yields a highly porous three-dimensional network of interconnected 18 nm nanocrystals.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

3
Tetramethyl rhodamine isothiocyanate (TRITC) was conjugated to NC YAG:Ce by using glycine as a bridging molecule.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

4
Efficient energy transfer is observed from the Ce3+ ions to the TRITC molecules upon photoexcitation of the NC YAG:Ce–TRITC conjugates.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

5
Resonant energy transfer (ET) between a donor in the excited state and a neighboring acceptor is commonly observed between dyes,1 quantum dots and dyes,2,3 quantum dots,4,5 and metal ions.6

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

6
The ET process can occur via several mechanisms, but the electric dipole–electric dipole interaction is the most effective one over relatively long distances (up to several tens of angstroms, with a r−6 distance dependence).6

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

7
This mechanism is particularly efficient when both donor and acceptor have allowed electric dipole resonant transitions.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

8
ET between suitable probes can be used to obtain structural information or monitor dynamic processes in biological systems but its usefulness is severely limited by fast photobleaching when dyes are used for tagging the target molecules.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

9
Semiconductor quantum dots are more stable than dyes7 but still suffer from photoinduced degradation processes.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met2

10
Luminescent ions in inorganic phosphors offer the prospect of a greatly superior photo-stability associated with high quantum yields.

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj1

11
Intermittence in emission (blinking), which is a problem in certain applications of single quantum dots, is not expected for inorganic phosphor nanocrystals as they contain a large number of luminescing ions (typically between 10 and 10 000 depending on the size of the nanocrystal and the concentration of luminescing ions).

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj1

12
In the past few years there has been a growing interest in the properties of nanocrystalline lanthanide-doped insulating phosphors, and several well-known materials, such as Y2O3:Eu3+,8–10 Lu2O3:Eu3+,11 YVO4:Eu3+,12 LaPO4:Ln3+ (Ln = Eu or Ce/Tb13), YbPO4:Er3+,14 and Y3Al5O12:Ln3+ Ln = Nd, Eu, Tb, Tm or Ce15–19), have been prepared in nanocrystalline form.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

13
The application of lanthanide-doped nanocrystalline phosphors as biological labels would however strongly limit the photon turnover rates due to the long (μs to ms) decay time of their intraconfigurational 4fn luminescence.

Type: Object | Advantage: No | Novelty: New | ConceptID: Obj1

14
One of the few exceptions for visible-emitting phosphors is Y3Al5O12:Ce (YAG:Ce).

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj1

15
In YAG:Ce the 5d state of Ce3+ is situated at a low energy resulting in a fast intraconfigurational 5d → 4f luminescence in the visible.

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj1

16
The luminescence life time of this emission is 65 ns.20

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj1

17
This luminescence decay time is ideally suited for time-gated detection of biological species, since it is slower than the autofluorescence background decay common to many biological species (typically a few ns), but is still fast enough to ensure a high turnover rate.

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj1

18
Additional attractive features are the excitation in the blue (which further decreases the autofluorescence background obtained under UV excitation) and the large Stokes shift of the luminescence.

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj1

19
The strong electric dipole of the Ce3+ f–d emission also makes NC YAG:Ce a promising candidate as donor for efficient ET processes to dyes.

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj1

20
In order to evaluate the potentiality of NC YAG:Ce as a probe in biological systems the ET from NC YAG:Ce to tetramethyl rhodamine isothiocyanate (TRITC) was investigated.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

21
TRITC is chemically conjugated to highly luminescent NC YAG:Ce by using glycine as a linker molecule.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

22
Steady state (excitation and emission spectra), time-domain (fluorescence decay times) measurements and photobleaching experiments clearly demonstrate the occurrence of ET from Ce3+ to TRITC molecules in the NC YAG:Ce–TRITC conjugates.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

23
Nanocrystalline YAG:Ce (0.5 mol%) was prepared by using a combustion synthesis technique similar to that reported in,10 but optimized to yield nanoparticles smaller than 20 nm with a small degree of sintering.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

24
Glycine was used as a fuel and the metal nitrates as oxidizers.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

25
Typically, 3 mmol of Ln(NO3)3·4H2O (Ln = Y and Ce), 5 mmol of Al(NO3)3·9H2O and 14 mmol of glycine were dissolved in 4 mL of demi-water and the resultant solution was dried overnight at 80 °C.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

26
The resulting transparent gel was placed into a preheated furnace at 650 °C for 30 min in air.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

27
The as-synthesized product was a foamy porous gray body, which was shown by X-ray powder diffraction to be amorphous.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

28
This product was subsequently treated at 1150 °C for 90 s under air, yielding a highly porous yellow body.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

29
Fig. 1 shows the X-ray powder diffraction patterns of the final product and of a microcrystalline powder sample of YAG:Ce (commercial phosphor purchased from Nichia Co.).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

30
It can be concluded that the final product consists of single phase crystalline YAG.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

31
The peak broadening and the small shift of the peak maxima to lower angles can be ascribed to a finite size effect, indicating that the sample consists of nanocrystalline domains.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

32
The mean size of these domains, determined from the full width at half maximum of the peak with the Debye–Scherrer formula21 is 18 nm.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

33
Thermal treatments at temperatures higher than 1150 °C and/or for longer times yield larger average nanocrystal sizes and more pronounced sintering.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

34
Temperatures below 1000 °C yield amorphous white samples (non-luminescent).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

35
The use of other fuels such as urea or citric acid leads to slightly larger YAG:Ce nanocrystals (23 nm), lower quantum yields and a larger degree of sintering.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

36
The NC YAG:Ce samples are highly friable and can be easily ground to a powder and broken apart into NC clusters (100–1000 nm) by sonic waves (sonic bath).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

37
The transmission electron microscopy (TEM) image of one such NC YAG:Ce cluster is depicted in Fig. 2A, which clearly shows that the NC YAG:Ce samples consist of a highly porous and interconnected three-dimensional network of nanoparticles.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

38
The high degree of crystallinity of these YAG:Ce nanoparticles is confirmed by both high-resolution TEM (Fig. 2B) and selected area electron diffraction (inset of Fig. 2A).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

39
The size of the YAG:Ce nanocrystals obtained from TEM and HR-TEM is consistent with the average size determined from XRD (∼20 nm).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

40
To conjugate TRITC to NC YAG:Ce a portion of 35 mg NC YAG:Ce (a slightly yellow powder) in 10 mL ultra-pure water was mixed thoroughly with 35 mg glycine (aminoacetic acid) and left overnight.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

41
Excess of glycine was removed by centrifuging and several times rinsing with ultra pure water.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

42
Glycine was chosen as the linker molecule because it can bind to both TRITC and NC YAG:Ce.

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met1

43
The carboxylic group of glycine is expected to strongly bind to the surface of the YAG:Ce nanocrystals via the Y3+ and Al3+ ions, whereas its primary amine group reacts with the isothiocyanate group of TRITC.

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met1

44
The use of TRITC as functionalised dye allows for selective excitation of the YAG:Ce at 440 nm since the absorption of TRITC at this wavelength is very low.

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met1

45
A disadvantage of TRITC as a dye is the spectral overlap between the emission of YAG:Ce and TRITC which complicates a quantitative analysis of the energy transfer efficiency from the emission spectra.

Type: Method | Advantage: No | Novelty: New | ConceptID: Met1

46
To the glycine capped NC YAG:Ce 35 μg of TRITC in 10 mL methanol was added and the sample was left overnight.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

47
The excess of TRITC was removed by centrifuging and rinsing twice with methanol.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

48
The sample (hereafter named TRITC-NC YAG:Ce conjugate) is a light pink powder and was dried in air prior to characterization.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

49
It should be mentioned that treatment of YAG:Ce with glycine and Rhodamine B (a dye molecule similar to TRITC, but without the isothiocyanate group) did not lead to a dye-NC YAG:Ce conjugate, showing that the presence of an isothiocyanate group is crucial for the formation of the conjugate.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

50
Steady-state emission and excitation spectra were recorded with a SPEX Fluorolog spectrofluorometer, model F2002, equipped with two monochromators (double-grating, 0.22 m, SPEX 1680) and a 450 W xenon lamp as the excitation source.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

51
Luminescence decay curves were measured using a Pico Quant PDL 800-B laser (λex = 440 nm, 2.5 MHz repetition rate, 55 ps pulse width) as an excitation source.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

52
The luminescence was dispersed by a monochromator (0.1 m focal length, 1350 line mm−1 grating, blazed at 500 nm) and detected by a fast Hamamatsu H5738P-01 photomultiplier tube.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

53
The decay curves were obtained by time correlated single photon counting (TCPCS) via time-to-amplitude conversion (TAC) with a Time Harp 100 computer card.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

54
The ratio of stop to start pulses was kept low (below 0.05) to assure good statistics.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

55
Fig. 3 shows the excitation spectra (A, λem = 580 nm) and emission spectra (B, λex = 440 nm) of tetramethyl rhodamine isothiocyanate (TRITC) in water (dark gray curves), NC YAG: Ce (light gray curves) and TRITC-NC YAG:Ce conjugate (black curves).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

56
The emission spectrum of NC YAG:Ce (0.5 mol%) consists of a broad band with maximum around 550 nm that is ascribed to a 5d → 4f transition.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

57
The 4f → 5d excitation transitions yield broad bands at around 260 nm (not shown), 340 nm and 450 nm.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

58
The band at 260 nm is very weak because the upper 5d states of Ce3+ in YAG have energies within the conduction band of the host and therefore excitation in these levels results mainly in quenching.20

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

59
The Ce3+ 5d level populated by the transition occurring at 340 nm is just below the YAG conduction band and will also be quenched to some extent at room temperature.20

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

60
There are no significant differences between the emission and excitation spectra of NC YAG:Ce and bulk YAG:Ce, as expected considering that the band-gap of YAG is very large (7 eV) and that the electronic transitions involved in the luminescence are localized on the Ce3+ ion.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

61
Quantum size effects, as observed for the luminescence of nanocrystalline semiconductors, are not expected for the luminescence of Ce3+ in NC YAG.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

62
The quantum efficiency of the NC YAG:Ce sample is lower than that of a bulk sample (∼50% at λex = 430 nm, instead of the 70% observed for commercial Nichia phosphor).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

63
The lower quantum efficiency of the nanocrystalline samples can be ascribed to a decrease in the excitation efficiency due to light scattering and to quenching and competitive absorption by surface defects (e.g. oxygen vacancies and/or disorder at grain boundaries20).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

64
The quantum efficiencies can be improved by longer thermal treatments or higher temperatures, which however lead to undesired nanocrystal growth and sintering.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

65
The luminescence quantum efficiency is given by the number of emitted photons divided by the number of absorbed photons, and was determined by comparison with standard phosphors, following the method of de Mello Donega et al.22

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

66
This method is accurate within 10%.

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met4

67
It is clear from the excitation spectra in Fig. 3A that selective excitation of NC YAG:Ce is possible at 440 nm (TRITC hardly absorbs at this wavelength).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

68
The black curve in Fig. 3B shows the emission spectrum of TRITC–NC YAG:Ce conjugate under 440 nm excitation.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

69
The weak band around 520 nm can be ascribed to Ce3+ emission, while the peak around 580 nm is due to TRITC emission.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

70
This provides a clear indication that ET from Ce3+ ions to TRITC takes place, since the TRITC emission dominates the emission spectrum despite its negligible absorption at 440 nm, whereas the strong absorption of YAG:Ce at 440 nm yields only a weak Ce3+ luminescence.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

71
We would like to point out that the spectral overlap between the emission of the donor and the absorption of the acceptor, needed for ET via dipole-dipole interaction, is good since the maximum of the donor (Ce3+) emission and the acceptor (TRITC) absorption band are both situated around 550 nm.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

72
In the excitation spectrum of TRITC–NC YAG:Ce conjugate (Fig. 3A, black curve) the peak at ∼550 nm is due to direct excitation of the TRITC.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

73
The peak around 450 nm is ascribed to direct excitation of Ce3+ ions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

74
It is noteworthy that the relative intensity of the peak around 375 nm for the TRITC–NC YAG:Ce conjugate is larger than expected by a simple convolution of the excitation bands of NC YAG:Ce and TRITC, suggesting an increased excitation efficiency due to resonance between the Ce3+ 5d level at 3.64 eV (340 nm) and TRITC excited states.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

75
This resonance would lead to efficient ET between the two levels, competing with the depopulation of the Ce3+ 5d level via the YAG conduction band.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

76
The observation of Ce3+ excitation bands in the excitation spectrum of the TRITC emission at 580 nm in the conjugate sample provides further evidence for the occurrence of Ce3+ → TRITC ET, but it is not unambiguous since the broad emission band of Ce3+ overlaps with the emission of TRITC.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

77
The luminescence decay curve of the Ce3+ emission provides a more direct way to probe whether Ce3+ → TRITC ET is indeed taking place.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

78
Fig. 4 shows the luminescence decay curves of NC YAG:Ce (in light gray) and of TRITC–NC YAG:Ce conjugate (in black).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

79
The decay curves are recorded monitoring luminescence at 520 nm to avoid detection of the TRITC emission.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

80
The decay curve of the Ce3+ ions in YAG is nearly monoexponential and has a decay time of 60 ns.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

81
This is very close to the reported decay time of Ce3+ ions in bulk YAG sample (65 ns,20).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

82
This is surprising since the effective refractive index is lower in NC YAG:Ce than in the bulk material due to the presence of a lower refractive index material (liquid helium or air) around the nanocrystals and within a distance of the order of the wavelength of light.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

83
It is expected that, through local field effects, the lower effective refractive index would reduce the radiative decay rate of the Ce3+ emission in NC YAG:Ce in comparison with the bulk.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp3

84
This effect has already been observed for NC Y2O3:Eu3+.23

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

85
In the case of NC YAG:Ce, however, the filling fraction is not known, what makes the comparison with the previous studies on NC Y2O3:Eu3+ difficult.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

86
Further, non-radiative processes are more important for the nanocrystal than for the bulk materials due to the larger surface to volume ratio, which is reflected in an initial deviation from single-exponential decay and a lower quantum efficiency.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp4

87
The slightly faster initial decay, observed for NC YAG:Ce can explain why the luminescence decay time measured, defined as the time at which the emission intensity has dropped to 1/e of its initial value, is shorter for the NC YAG:Ce despite the lower effective refractive index.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp4

88
For the TRITC–NC YAG:Ce conjugate (black curve) a fast initial decay in the luminescence intensity of the Ce3+ ion is observed, indicating the presence of an additional decay channel for the Ce3+ ions in the conjugate, which can be ascribed to resonant ET from the 5d excited states of Ce3+ in YAG to acceptor excited states of TRITC molecules at the nanocrystals surface.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

89
Radiative transfer from the Ce3+ ions to the TRITC molecules may also occur, but would not lead to shorter life times for the donor states.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp5

90
The fact that coupling to TRITC results in significantly faster luminescence decay curve shows that non-radiative energy transfer via dipole–dipole interaction is dominant.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

91
The average ET rate can be estimated from the decay curves and is ∼108–109 s−1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

92
Since both the Ce3+ and the TRITC emissions are allowed electric-dipole transitions, the dipole–dipole interaction will be strong allowing efficient ET to occur if the Ce3+–TRITC distances are sufficiently small.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp6

93
Using the Förster–Dexter theory for energy transfer via dipole–dipole interaction an estimate of the energy transfer rate can be made.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

94
The critical distance for energy transfer (where the radiative rate equals the energy transfer rate) is given by24R6c = 3 × 1012fAE−4SOHere fA is the oscillator strength of the acceptor transition, E is the energy at the maximum of spectral overlap (in eV) and SO is the spectral overlap integral.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

95
The oscillator strength as determined from the TRITC absorption spectrum is about 0.5.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

96
The spectral overlap determined from the YAG:Ce emission and TRITC absorption spectrum is 2 eV−1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

97
This gives a critical distance for energy transfer from Ce3+ to TRITC of about 7 nm.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

98
As a result, energy transfer rates of 108 s−1 and faster are expected for Ce3+ ions in an outer shell of 5 nm thick in the NC.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp7

99
This is quite consistent with the observations.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

100
The luminescence decay curves show that a majority of the Ce3+ show a fast decay due to efficient energy transfer to TRITC.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

101
For the average size of 18 nm YAG:Ce nanocrystals more than 90% of the Ce3+ ions will be within a 5 nm distance from the surface and will transfer the excitation energy with transfer rates of 108 s−1 and higher to TRITC molecules at the surface.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

102
It should be noted that a 18 nm diameter YAG:Ce (0.5 mol%) nanocrystal contains about 1700 Ce3+ ions statistically distributed over all available Y3+ sites.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

103
This will cause a distribution in the TRITC to Ce3+ distances.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp8

104
Since ET by electric dipole-electric dipole interaction scales with r−6, Ce3+ ions closer to the surface will transfer their energy much more efficiently then ions in the core of the nanocrystal.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp8

105
This distribution of donor-acceptor distances will result in a distribution of ET rates between Ce3+ ions and TRITC molecules.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp8

106
The size polydispersity inherent to the NC YAG:Ce samples further broadens the distribution of ET rates.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp9

107
Therefore only an average ET rate can be determined from the experimental data.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

108
ET is not only possible between the Ce3+ ions and the TRITC molecules but also between Ce3+ ions.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp10

109
Therefore Ce3+ ions deep inside the nanocrystals may also transfer their energy to Ce3+ ions closer to the surface which subsequently transfer their excitation energy to the TRITC molecules, thus increasing the ET rate.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp10

110
A more quantitative analysis of the energy transfer process is not possible in view of the various uncertainties discussed above.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

111
Photobleaching can be used as a control experiment25 to show that the decay time shortening of the Ce3+ luminescence observed in the TRITC-YAG:Ce conjugate is indeed due to energy transfer.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met6

112
TRITC will degrade after being exposed to UV light for a long period, while YAG:Ce is photostable.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp11

113
Fig. 5 shows the excitation (A) and emission (B) spectra of the TRITC-YAG:Ce conjugate prior to (dark gray) and after (black) 65 hours of irradiation with 300 nm light from a 450 W xenon lamp.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

114
The color of the sample after irradiation changed from pink to slight yellow, clearly indicating that the photodecomposition of the TRITC molecules has occurred.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

115
In the excitation spectrum of the photobleached sample (Fig. 5A) the peak around 550 nm (due to direct excitation of TRITC) is strongly reduced, which implies that the amount of TRITC in the sample has decreased.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

116
However, not all the TRITC molecules were bleached, despite the long period of UV radiation, possibly because the strong scattering characteristic of nanoporous materials limits the penetration depth of the light.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

117
In the emission spectra (Fig. 5B) a strong decrease in the emission intensity of the peak around 580 nm (the TRITC emission) is observed after bleaching.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

118
Since less TRITC is present at the surface, the average distance between Ce3+ ions and TRITC molecules increases.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp12

119
Therefore the likelihood of Ce3+ to TRITC energy transfer is diminished and the fraction of radiative recombination of the Ce3+ ions increases.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp12

120
Both contributions will be reflected in the decay curves of the photobleached TRITC–YAG:Ce conjugate sample (Fig. 4, dark gray curve).

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp12

121
The fast initial decay is still present, but has a much smaller contribution than prior to the photobleaching, indicating that a smaller population of Ce3+ ions have a TRITC molecule at distances short enough to allow efficient ET to take place.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

122
This is also reflected in the larger contribution of the exponential decay tail due to the radiative decay of the Ce3+ ions without neighboring TRITC molecules.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

123
In conclusion, we have demonstrated that efficient energy transfer (ET rates of ∼108–109 s−1) from Ce3+ ions to TRITC molecules occurs in the TRITC-NC YAG:Ce conjugate.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

124
Also, we have shown that nanocrystalline YAG:Ce can be successfully conjugated to organic molecules, therefore establishing its potentiality as a biolabel.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

125
Additional experiments aiming at imaging and spectrally resolving single YAG:Ce nanocrystals have also been successfully carried out and will be reported elsewhere.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3