light 
 upconversion is the generation of high - energy photons from 
 low - energy photons , for example , the conversion of red light to blue 
 light . generating upconverted light can be achieved using different 
 systems such as two - photon absorption dyes , rare earth - doped materials 
 or nanoparticles , and triplet  triplet annihilation ( tta - uc ) . 
 among these systems , tta - uc offers many advantages : it works at low 
 excitation power ( down to 1 mw cm ) , it uses sensitizers 
 having high molar absorptivity , and the obtained upconversion quantum 
 yields are high , typically 15% in aqueous solution . since its popularization more than a decade ago , tta - uc has been used in many applications such 
 as photocatalysis , solar energy harvesting , drug delivery and activation , and luminescence bioimaging . tta - uc is based 
 on the photophysical interplay of photosensitizer and annihilator 
 chromophores ( see figure s1 ) . the photosensitizer absorbs low energy light , 
 after which intersystem crossing leads to a long - lived triplet state . the energy of this triplet state is transferred to the annihilator 
 upon diffusional collision by means of triplet  triplet energy 
 transfer ( ttet ) ; a succession of ttet leads to a concentration buildup 
 of long - lived triplet - state annihilators . triplet annihilation upconversion , 
 in which one of them departs with the energy of both triplet states , 
 to reach a high - energy singlet state . finally , this singlet excited 
 state returns to the ground state by emission of a high - energy photon , 
 thus realizing light upconversion . tta - uc has been demonstrated 
 in an extensive assortment of organic , 
 inorganic , and/or supramolecular materials , as 
 well as in nano- or microsized particles . among the various applications of tta - uc , some of them require to 
 operate above room temperature , such as bioimaging and phototherapy . because ttet and tta occur via molecular collisions , these 
 processes are highly dependent on molecular diffusion ; the efficiency 
 of tta - uc was reported as being greatly influenced by the fluidity 
 of the matrix containing the dyes , and hence by the temperature . for many materials , a higher temperature leads 
 to a higher fluidity , and therefore to higher tta - uc efficiency . for 
 example , green - to - blue tta - uc in a rubbery polymer matrix was only 
 visible above the glass transition temperature of the material , where 
 the matrix becomes more fluid . however , 
 diffusion is not the only important factor . first of all , temperature - dependent 
 chemical phenomena such as dye aggregation may affect upconversion 
 as well : counterintuitively , it was recently shown that at lower temperatures , 
 mixed aggregation of sensitizer and annihilator molecules in diluted 
 conditions resulted in higher tta - uc efficiency . it has also been shown that upconversion in gel matrices 
 decreased at higher temperatures due to temperature - dependent disassembly 
 of the host material . overall , understanding 
 the temperature dependence of all chemical and physical properties 
 of a given matrix is necessary for optimizing upconversion . our group recently demonstrated that green - to - blue and red - to - blue 
 tta - uc can be realized in the phospholipid membrane of neutral pegylated 
 liposomes composed of 1,2-dimyristoyl - sn - glycero-3-phosphocholine 
 ( dmpc ) . this knowledge was later used for the activation of photoactivatable 
 chemotherapeutic agents in the photodynamic window . in our initial studies it was reported that the upconversion 
 intensity was reversibly affected by changes in temperature . upon heating the sample from 15 to 25 c the upconversion intensity increased significantly , which we interpreted 
 as a consequence of the gel - to - liquid crystalline phase transition 
 temperature ( tm ) of the 
 dmpc lipid bilayer . upon raising the temperature above tm the molecular diffusion of the dyes 
 in the membrane is expected to increase greatly , which should lead 
 to higher ttet and tta rates , and thus higher tta - uc efficiencies . 
 in this work , we systematically investigated the temperature dependency 
 of tta - uc in neutral pegylated liposomes made of different lipids 
 with different transition temperatures tm , to optimize the lipid composition of red - to - blue 
 tta - uc drug - delivery systems functioning at human body temperature . palladium tetraphenyltetrabenzoporphyrin ( 1 ) was purchased from bio - connect ( huissen , the netherlands ) . perylene ( 2 ) was purchased from sigma - aldrich chemie 
 bv ( zwijndrecht , the netherlands ) . all lipids were purchased from 
 either lipoid gmbh ( ludwigshafen , germany ) or avanti polar lipids 
 ( alabaster , al , usa ) and stored at 18 c . phosphate buffered saline ( dpbs ) was purchased from sigma - aldrich 
 and had a formulation of 8 gl nacl , 0.2 
 gl kcl , 0.2 gl kh2po4 , and 1.15 gl k2hpo4 with a ph of 7.17.5 . all liposome formulations were prepared 
 by the classical hydration - extrusion method . as an example , the preparation 
 of liposome sample o12 is described here . aliquots of 
 chloroform stock solutions containing the liposome constituents were 
 added together in a flask to obtain a solution with 5.0 mol 
 dopc , 0.20 mol dspe - mpeg-2000 , 2.5 nmol compound 1 , and 25 nmol compound 2 . the organic solvent was removed 
 by rotary evaporation and subsequently under high vacuum for at least 
 30 min to create a lipid film . 1.0 ml dpbs buffer , with or without 
 0.3 m sodium sulfite , was added and the lipid film was hydrated by 
 4 cycles of freezing the flask in liquid nitrogen and thawing in warm 
 water ( 60 c ) . the resulting dispersion was extruded through 
 a whatman nuclepore 0.2 m polycarbonate filter at least 10 
 c above the main phase transition temperature of the lipid for 
 at least 11 times using a mini - extruder from avanti polar lipids , 
 inc . ( alabaster , alabama , usa ) , fitted with two 1001rn gastight syringes 
 from hamilton ( bonaduz , switzerland ) . warning : heating the gastight 
 syringes to 5070 c will cause the teflon plunger to 
 leak at room temperature  it is advised to use one set of syringes 
 for hot extrusion only ! the number of extrusions was always odd to 
 prevent any unextruded material ending up in the final liposome sample . the extrusion filter remained practically colorless after extrusion , 
 suggesting near - complete inclusion of the dyes in the lipid bilayer . liposomes were stored in the dark at 4 c and used within 7 days . the average liposome size and polydispersity index were measured with 
 a malvern instruments zetasizer nano - s machine , operating with a wavelength 
 of 632 nm . differential scanning 
 calorimetry ( dsc ) was performed on a ta instruments ( de , usa ) nano - dsc 
 iii instrument in the range of 5 to 50 c with a scanning rate 
 of 1 c min at 3 atm . the capillary cell 
 ( v = 300 l ) was filled with the liposome solution 
 ( lipid bulk concentration of 5 mm ) , and the reference cell was filled 
 with pbs buffer solution . the liposome dispersions were degassed for 1015 min 
 prior to measurement on a nalgene degassing station . for each sample , at least two cycles of heating and cooling were performed with 10 
 min of thermal equilibration between the ramps . the machine was cleaned 
 beforehand with 50% formic acid and rinsed thoroughly with milli - q 
 water . absorption and 
 emission spectroscopy was conducted in a custom - built setup ( figure s2 ) . all optical parts were connected 
 with fc - uvxxx-2 ( xxx = 200 , 400 , 
 600 ) optical fibers from avantes ( apeldoorn , the netherlands ) , with 
 a diameter of 200600 m , respectively , and that were 
 suitable for the uv  vis range ( 200800 nm ) . typically , 
 2.25 ml of sample was placed in a 111-os macro fluorescence cuvette 
 from hellma in a cuv - uv / vis - tc temperature - controlled cuvette holder 
 with stirring from avantes . deoxygenated toluene samples were prepared 
 in a glovebox in a sealed fluorescence cuvette . the cuvette holder 
 temperature was controlled with a tc-125 controller and t - app computer 
 software from quantum northwest ( liberty lake , wa , usa ) , while the 
 sample temperature was measured with an omega rdxl4sd thermometer 
 with a k - type probe submerged in the sample . the sample was excited 
 with a 10 mw collimated 630 nm laser light beam ( 4 mm beam diameter , 
 80 mw cm ) from a diomed 630 nm pdt laser . the 
 630 nm light was filtered through a 630 nm band - pass filter ( fb63010 
 from thorlabs , dachau / munich , germany ) put between the laser and the 
 sample . the excitation power was controlled using the laser control 
 in combination with a ndl-25c-4 variable neutral density filter ( thorlabs ) , 
 and measured using a s310c thermal sensor connected to a pm100usb 
 power meter ( thorlabs ) . vis absorption spectra were measured 
 using an avalight - dhc halogen - deuterium lamp ( avantes ) as light source 
 and a 2048l starline spectrometer ( avantes ) as detector , both connected 
 to the cuvette holder at a 180 angle and both at a 90 
 angle with respect to the red laser irradiation direction . the filter 
 holder between cuvette holder and detector was in a position without 
 a filter ( figure s2 , item 8) . luminescence 
 emission spectra were measured using the same detector but with the 
 uv  vis light source switched off . to visualize the spectrum 
 from 450 to 950 nm , while blocking the red excitation light , a thorlabs 
 nf-633 notch filter was used in the variable filter holder . all spectra 
 were recorded with avasoft software from avantes and further processed 
 with microsoft office excel 2010 and origin pro 9.1 software . temperature 
 dependent luminescence experiments were done with continuous irradiation 
 and temperature ramping , except for phosphorescence measurements of 
 compound 1 to prevent bleaching during the experiment . instead , spectra were taken every 5 c with 10 min thermal equilibration 
 between temperature points . the absolute quantum yield of upconversion was determined by means 
 of an integrating sphere setup . neutral pegylated liposome dispersions 
 were prepared in phosphate 
 buffered saline ( pbs ) by hydration and extrusion of lipid films containing 
 six different neutral phosphatidylcholines , i.e. , 1,2-dioleyl - sn - glycero-3-phosphocholine ( dopc ) , 1,2-dilaureyl - sn - glycero-3-phosphocholine ( dlpc ) , 1,2-dimyristoyl - sn - glycero-3-phosphocholine ( dmpc ) , 1,2-dipentadecanoyl - sn - glycero-3-phosphocholine ( dpdpc ) , 1,2-dipalmitoyl - sn - glycero-3-phosphocholine ( dppc ) , and 1,2-distearoyl - sn - glycero-3-phosphocholine ( dspc ) and in the presence of 
 4 mol % of sodium n-(carbonyl - methoxypolyethylene 
 glycol-2000)-1,2-distearoyl - sn - glycero-3-phosphoethanolamine 
 ( dspe - mpeg-2000 , see figure 1 ) . addition of dspe - mpeg-2000 is a well - known strategy to 
 prevent liposome aggregation and fusion , and moreover increases the hydrophobic dye loading capacity of phospholipid 
 membranes . the lipid composition of liposome 
 samples o , l , m , pd , p , and s is shown in table 1 . a well - investigated red - to - blue 
 tta - uc dye couple consisting of palladium tetraphenyltetrabenzoporphyrin 
 ( 1 ) and perylene ( 2 , see figure 1 ) was selected for incorporation 
 in the lipid bilayer of the liposomes . samples containing these dyes , 
 i.e. , o12 , l12 , m12 , pd12 , p12 , and s12 ( defined in table 1 ) , were prepared following 
 an identical procedure . the hydrodynamic diameters ( z - ave = 137  6 nm ) and polydispersity indices ( pdi = 0.09  
 0.02 ) , as measured by dynamic light scattering ( dls ) , were found to 
 be very similar regardless of the lipid type or dye concentration . chemical 
 structures of dopc , dlpc , dmpc , dpdpc , dppc , dspc , dspe - mpeg-2000 , 
 palladium tetraphenyltetrabenzoporphyrin ( 1 ) , and perylene 
 ( 2 ) . dsc measurements 
 were performed 
 with a scanning rate of 1 c min at 3 atm 
 pressure . all liposomes 
 were prepared with 
 5.0 mm lipid and 0.20 mm dspe - mpeg-2000 in pbs ( without sulfite ) . compound 2 was 
 incorporated 
 at 0.5 mol % with respect to the phospholipid ; higher dye contents 
 could not be reproducibly obtained with our liposome preparation method . tm is defined as the main transition temperature of 
 the bilayer , 
 and h as the molar enthalpy change of the 
 phase transition ( the enthalpy change of the pretransition is included , 
 in case there is one ) . it is well - known that phase 
 changes of phospholipid membranes greatly 
 influence the two - dimensional translational molecular diffusion coefficient 
 ( dt in m s ) of membrane solutes . therefore , the gel - to - liquid phase transition 
 temperature ( tm ) and 
 the total enthalpy change of the phase transition ( h ) were measured for samples based on dmpc , 
 dpdpc , dppc , and dspc using differential scanning calorimetry ( dsc , 
 see table 1 , figure 2b , and figure s3 ) . tm and h for dye - free 
 pegylated liposomes m , pd , p , and s were found to be very close to literature values 
 for peg - free liposomes , i.e. , the peg groups did not significantly 
 influence the phase transition at these concentrations . upon functionalization 
 of the pegylated liposomes with compounds 1 and 2 , a small decrease in the main transition peak height was 
 observed , but the main features of the thermogram remained . these 
 results indicate that for liposome samples m12 , pd12 , p12 , and s12 compounds 1 and 2 were indeed buried in the lipid bilayer , 
 and that their presence only minimally perturbed the physical properties 
 of the membranes . no transitions were found between 5 and 50 c 
 for samples o , o12 , l , and l12 , because tm for pure dopc and dlpc are reported to be below the freezing point 
 of water . ( a ) 
 typical absorption ( solid , left axis ) and emission spectrum ( dashed , 
 right axis , exc = 630 nm , intensity 80 mw cm ) of l12 liposomes ( [ dlpc ] = 1.0 mm , 
  = 0.5 m ,   = 5 m ) at 
 20 c in 0.3 m sodium sulfite pbs under air . ( b ) differential 
 scanning calorimetry thermograms between 5 and 50 c of liposomes 
 with tta - uc dyes ( o12 , l12 , m12 , pd12 , p12 , and s12 , solid ) 
 or without ( o , l , m , pd , p , and s , dashed ) . the thermograms 
 for liposomes s and s12 were recorded between 
 35 and 65 c ( figure s3 ) . measurements were performed in heating mode with a scanning 
 rate of 1 c min at 3 atm pressure . ( c , d ) 
 temperature evolution of the upconversion quantum yield ( uc , c ) and of the residual sensitizer phosphorescence quantum 
 yield ( p , d ) of o12 , l12 , m12 , pd12 , p12 , and s12 . samples were heated from 5 to 50 c at a rate of 
 1 c min while continuously irradiated with 
 80 mw cm 630 nm light , at 1.0 mm lipid and   = 0.5 m and   = 5 m . next , uv  vis absorption 
 and emission spectroscopy was performed 
 on samples o12 , l12 , m12 , pd12 , p12 , and s12 at 20 c 
 in the presence of 0.3 m sodium sulfite ( figure 2 ) . in these samples , earlier work showed 
 that despite the large increase in buffer ionic strength , na2so3 does not affect the formation of dopc and dmpc upconverting 
 liposomes and allows stable tta - uc to occur in air . moreover , preliminary studies indicated that a 1:10 dye 
 ratio ( 1:2 = 0.05:0.50 mol % with respect 
 to the lipid ) resulted in optimized upconversion in m12 , while higher dye - loading was severely limited by the solubility 
 of 2 in the membrane . the absorption spectra of these 
 samples showed the superposition of the characteristic bands of 1 at 440 and 630 nm and the vibronically structured band of 2 from 350450 nm . upon 
 irradiation with 630 nm laser light ( 10 mw , 80 mw cm ) , phosphorescence of 1 at 800 nm and upconversion emission 
 of 2 at 474 nm were observed for each sample . the emission 
 stability at 20 c was tested for each formulation by continuously 
 irradiating for 1 h and collecting emission spectra . all samples exhibited 
 good emission stability during this period ( figure s4 ) . the absolute quantum yield of upconversion ( uc , 420610 nm ) and residual sensitizer phosphorescence 
 ( p , 725950 nm ) , defined by the number of 
 emitted photons divided by the number of absorbed photons ( supporting information ) , was determined by means 
 of an integrating sphere setup at room temperature ( 21.3 c ) . uc had values of 3.6% , 2.8% , 2.0% , 0.7% , 0.6% , and 
 0.3% for o12 , l12 , m12 , pd12 , p12 , and s12 , respectively . who 
 show that the upconversion intensity of green - to - blue tta - uc in liposomes 
 decreases strongly when going from dopc to dmpc , while no upconversion 
 was observed at all in dspc . thus , the 
 fact that we observe tta - uc in the long - chained saturated phospholipids 
 ( i.e. , dpdpc , dppc , dspc ) at room temperature is interesting in itself . this may be explained by the longer triplet lifetimes ( t ) of sensitizer 1 ( t = 250 
 s in dmf ) compared to the green - absorbing 
 sensitizer platinum octaethylporphyrin ( ptoep , t = 50 s in toluene ) , which increases 
 the possibility for tta - uc . to investigate the temperature dependency 
 of tta - uc in o12 , l12 , m12 , pd12 , p12 , and s12 , these samples 
 were heated from 5 to 50 c 
 at a rate of 1 c.min while stirring , and 
 upconversion spectra were continuously recorded . the same samples 
 were used as for the quantum yield determination  these measurements 
 were conducted within 24 h of each other . figure 2 shows the evolution 
 of uc and p vs temperature for 
 each liposome formulation . these curves were obtained by recording 
 the intensity of phosphorescence ( ip ) 
 and upconversion ( iuc ) vs temperature 
 and scaling these to the measured absolute quantum yields at 21.3 
 c ( vide supra ) . for o12 and l12 , both 
 uc and p gradually decreased with 
 increasing temperature . for m12 , pd12 , and p12 , uc increased up to 25 , 33 , and 42 c , 
 respectively , and then decreased gradually , whereas p decreased steeply up to 25 , 34 , and 42 c , respectively , and 
 then continued to decrease , but less steeply . for s12 , uc increased and p decreased 
 with increasing temperature . when the samples were brought back from 
 50 to 5 c , the initial emission spectra at 5 c were obtained 
 again in all cases ( figure s5 ) and the 
 uv  vis absorption spectra were identical to those obtained 
 at the beginning of these experiments ( figure s6 ) ; both findings showing that bleaching did not occur and 
 that the thermophotophysical evolution is reversible . the shape of 
 the observed temperature behavior was nearly identical at 5 
 lower concentration for m12 , showing that the influence 
 of liposome scatter is negligible ( figure s7 ) . also , in a control experiment in which dspe - mpeg-2000 was omitted 
 from sample m12 , the shape of the observed temperature 
 behavior was very similar , which indicates that pegylation did not 
 have significant influence on the observed thermophotophysical behavior 
 ( figure s8 ) . interestingly , for m12 , pd12 , and p12 , the temperature values 
 at which uc maximizes and p kinks 
 are very close to the phase transition temperature of the bilayer 
 ( tm ) recorded with dsc . the increase of uc when approaching tm is easily explained : heating the liposomes 
 below tm greatly increases 
 the membrane fluidity and thus increases the lateral diffusion coefficient 
 ( dt ) of membrane dyes , which in turn causes 
 an increase in tta - uc efficiency . for instance , the dt for fluorescent probes in dmpc lipid bilayers has been 
 reported to increase from 0.01 m s at 15 c to 6 m it is worth mentioning that for such dmpc 
 bilayers , the foremost change in dt ( a 
 three - order increase in magnitude ) was found between 20 and 25 c , and so the most considerable transition in tta - uc 
 efficiency was expected to occur in this temperature domain . this 
 is indeed in accordance with our data for m12 . in the 
 absence of accurate literature data of dt in dpdpc and dppc across the full temperature range , we assume that 
 the same explanation holds for the results obtained with pd12 and p12 . however , this rationale is clearly no longer 
 valid above tm : although dt continues to increase ( vide supra ) , uc decreased . furthermore , for o12 and l12 , in absence of a phase transition between 5 and 50 c , uc and p both decrease across the entire 
 temperature range . it is thus clear that other photophysical phenomena 
 must play a role in the temperature dependence of tta - uc in lipid 
 bilayers . therefore , the thermophotophysical behavior of the 
 isolated dyes 
 was considered in dopc , dmpc , and toluene ( figure 3 ) . first , the fluorescence intensity of compound 2 ( exc = 420 nm , em = 
 474 nm ) was found to decrease by 10% in both dopc liposomes and toluene 
 when heated from 5 to 50 c . this is most likely explained by 
 a slightly increased thermal deactivation . in dmpc , the fluorescence 
 intensity increased by 25% when heated from 5 to 30 c , with 
 the most sharp increase around 25 c , and then decreased slightly 
 again up to 50 c . in all three systems , no significant spectral 
 fluorescence differences are observed between 5 and 50 c ( figure s9 ) . this observation is in agreement 
 with the work of khan et al . , who reported that perylene tends to 
 form staggered nonfluorescent aggregates in the tightly packed gel 
 membrane below tm , which 
 break apart in the more loosely packed liquid - crystalline state above tm . since the fluorescence intensity is lower in the presence of such 
 aggregates , the tta - uc efficiency is lower below tm . overall , dissociation of perylene 
 aggregates gives an additional explanation for the increase of upconversion 
 intensity up to tm . temperature - dependent 
 emission spectroscopy of compounds 2 or 1 in toluene , dmpc liposomes , or dopc liposomes . ( a ) normalized fluorescence intensity at 474 nm of compound 2 in toluene 
 ( dashed , 20 m ) , m2 liposomes ( purple , [ dmpc ] = 
 1 mm ) , or o2 liposomes ( black , [ dopc ] = 1 mm ) as a function 
 of temperature . ( b ) temperature variation of the normalized phosphorescence 
 intensity at 800 nm in 5 c intervals for compound 1 in toluene under argon ( open circles ) and for liposomes o1 ( black triangles , [ dopc ] = 1 mm ) or m1 ( purple squares , 
 [ dmpc ] = 1 mm ) prepared in pbs with 0.3 m sodium sulfite . second , the phosphorescence intensity of 1 ( exc = 630 nm , em = 
 800 nm ) was investigated 
 under deoxygenated conditions . in toluene solution , roughly 50% of 
 the phosphorescence intensity is lost upon going from 5 to 50 c 
 due to increased thermal deactivation . when the dye was inserted into 
 dopc or dmpc liposomes ( o1 and m1 , respectively ) about 70% phosphorescence intensity was lost upon going from 5 to 
 50 c ; the additional 20% loss of phosphorescence intensity with 
 respect to the toluene sample may be due to increased dynamic self - quenching , 
 because the molecules are much more confined in the lipid bilayer . the explanation of self - quenching is supported by the fact that , for m1 , the highest loss of phosphorescence is observed around 
 the transition temperature , at which the fluidity of the membrane 
 increases most rapidly and diffusion - based processes such as self - quenching 
 are expected to have an increased effect . overall , these results explain 
 that the decrease of tta - uc with rising temperature is most likely 
 due to increased thermal deactivation and self - quenching of 1 . based on these data , we explain the typical maximization 
 of uc around tm in 
 lipid bilayers that have a transition temperature between 5 and 50 
 c as follows . on one hand , the increase in photosensitizer quenching 
 as a function of temperature is rather linear ( figure 3 ) . on the other hand , the temperature dependence 
 of dt has been described in the literature 
 as sigmoidal , with three orders of magnitude increase when approaching tm , and flattening directly 
 after tm . in other words , upon approaching tm the membrane becomes fluid rather quickly , 
 but once it reaches the liquid crystalline state the fluidity changes 
 negligibly . therefore , above tm the effect of the only minor increase in lateral diffusion 
 coefficient on the upconversion efficiency is completely outcompeted 
 by the increased quenching of the photosensitizer . furthermore , the 
 dissociation of annihilator aggregates results in a rather abrupt 
 and significant increase in fluorescence around tm as well ( figure 3a ) . it is thus concluded that the combination 
 of these three temperature - dependent phenomena results in the maxima 
 that were observed in the uc versus temperature 
 curve at 25 , 33 , and 42 c for samples m12 , pd12 , and p12 , respectively ( figure 2c ) . finally , for the 
 biological application of these upconverting liposomes 
 in bioimaging or phototherapy , it would be beneficial to achieve the 
 highest upconversion intensity at human body temperature ( 37 c ) . 
 from our results , it is evident that the systems o12 , l12 , and m12 achieve similar upconversion quantum 
 yields at 37 c , while pd12 , p12 , and s12 exhibit lower quantum yields . altogether , the results 
 suggest that even though uc maximizes around tm ( for m12 , pd12 , and p12 ) , choosing a lipid with a tm near 37 c does not 
 result in an optimized upconverting liposome formulation . finally , 
 considering that little has been reported about the biocompatibility 
 of dlpc , we conclude that o12 and m12 upconverting 
 liposomes are the most promising for biological applications . the temperature dependence of red - to - blue tta - uc was studied in 
 pegylated liposomes with pc lipids with different lipophilic chain 
 lengths and transition temperatures , and it was found that the upconversion 
 efficiency maximizes around the order  disorder transition temperature 
 of the membrane , tm . three major effects contribute to this temperature dependency : ( 1 ) 
 an increase in lipid bilayer fluidity above tm results in higher diffusion rates and thus 
 in higher rates of ttet and tta and higher upconverted intensity ; 
 ( 2 ) perylene aggregates dissociate when t approaches tm , which results in higher 
 annihilator emission intensity ; and ( 3 ) higher thermal deactivation 
 and self - quenching rates of the photosensitizer at higher temperatures 
 lead to a lower ttet rate and lower upconversion intensity beyond tm . measuring the point at which iuc maximizes may be exploited for probing the 
 transition temperature of phospholipid membranes . furthermore , for 
 tta - uc applications that require high performance at elevated temperatures , 
 the results underline the importance of selecting photosensitizers 
 that are minimally affected by temperature . finally , the upconverted 
 intensity in dopc , dlpc , and dmpc liposomes were very similar at 37 
 c , which highlights that not being at the optimum temperature 
 for a given lipid composition does not necessarily mean that the upconverted 
 intensity is lower than when being at the optimum temperature for 
 another lipid composition . overall , tta - uc in liposomes can be realized 
 with many different lipids of different tm , and for both saturated and unsaturated lipids . 
 for applications in bioimaging and phototherapy , the phospholipid 
 can be rather freely chosen among dlpc , dmpc , and dopc , while dppc , 
 dpdpc , and dspc lead to slightly lower upconverted intensities . such 
 versatility allows for further optimizing the liposomal formulation 
 in terms of other properties such as stability to medium , biocompatibility , 
 toxicity , clearance from the bloodstream , and/or surface functionalization .
understanding the 
 temperature dependency of triplet 
 triplet 
 annihilation upconversion ( tta - uc ) is important for optimizing biological 
 applications of upconversion . here 
 the temperature dependency of red - to - blue 
 tta - uc is reported in a variety of neutral pegylated phospholipid 
 liposomes . in these systems a delicate balance between lateral diffusion 
 rate of the dyes , annihilator aggregation , and sensitizer self - quenching 
 leads to a volcano plot , with the maximum upconversion intensity occurring 
 near the main order 
 disorder transition temperature of the 
 lipid membrane .