Effect of short-term intensive design-based STEM learning on executive function: an fNIRS study of the left-behind children.

Design-based STEM learning is believed to be an effective cross-disciplinary strategy for promoting children's cognitive development. Yet, its impact on executive functions, particularly for disadvantaged children, still need to be explored. This study investigated the effects of short-term intensive design-based STEM learning on executive function among left-behind children. Sixty-one Grade 4 students from a school dedicated to the left-behind children in China were sampled and randomly assigned to an experimental group (10.70 ± 0.47 years old, n = 30) or a control group (10.77 ± 0.43 years old, n = 31). The experimental group underwent a two-week design-based STEM training program, while the control group participated in a 2-week STEM-related reading program. Both groups were assessed with the brain activation from 4 brain regions of interest using functional near-infrared spectroscopy (fNIRS) and behavioral measures during a Stroop task before and after the training. Analysis disclosed: (i) a significant within-group time effect in the experimental group, with posttest brain activation in Brodmann Area 10 and 46 being notably lower during neutral and word conditions; (ii) a significant between-group difference at posttest, with the experimental group showing considerably lower brain activation in Brodmann Area 10 and Brodmann Area 46 than the control group; and (iii) a significant task effect in brain activity among the three conditions of the Stroop task. These findings indicated that this STEM learning effectively enhanced executive function in left-behind children. The discrepancy between the non-significant differences in behavioral performance and the significant ones in brain activation implies a compensatory mechanism in brain activation. This study enriches current theories about the impact of Science, Technology, Engineering, and Mathematics (STEM) learning on children's executive function development, providing biological evidence and valuable insights for educational curriculum design and assessment.

Association between stress hyperglycemia ratio and functional outcomes in patients with acute ischemic stroke.

OBJECTIVE: This study aimed to evaluate the association between stress hyperglycemia ratio (SHR) and poor functional outcomes at 90 days in patients with acute ischemic stroke (AIS). METHODS: This study retrospectively collected 1988 AIS patients admitted to two hospitals in the Shenzhen area between January 2022 and March 2023. A total of 1255 patients with Fasting Blood-glucose (FBG) and hemoglobin A1c (HbA1C) values at admission were included in this analysis. SHR, measured by FBG/HbA1C, was evaluated as both a tri-categorical variable (Tertile 1: ≤ 0.83; Tertile 2: 0.84 -0.95; Tertile 3: ≥ 0.96). The outcome was poor functional outcomes (modified Rankin Scale [mRS] score 2-6) at 90 days. We performed univariate analysis, multiple equation regression analysis, stratified analysis, and interactive analysis. RESULTS: Compared with patients in the lowest tertile of SHR, the highest tertile group had significantly lower odds of achieving poor functional outcomes (adjusted odds ratio, OR = 2.84, 95% CI: 2.02-3.99, P < 0.0001) at 90 days after adjusting for potential covariates. Similar results were observed after further adjustment for white blood cell count, neutrophil count, lymphocyte count, fasting blood glucose, stroke type, intravenous thrombolytic therapy, baseline Glasgow score, and baseline NIHSS score. CONCLUSION: SHR, as measured by the FBG/HbA1C, was associated with an increased odds of achieving poor functional outcomes in patients with AIS at 90 days.

The nonlinear relationship between triglyceride glucose-waist circumference and stroke risk in middle-aged and elderly people: a nationwide prospective cohort study of the CHARLS.

BACKGROUND AND AIMS: Numerous research have focused on the relationship of metabolic markers and stroke risk, yet limited research has focused on the triglyceride glucose-waist circumference (TyG-WC) index. This study explored the possible association of TyG-WC and stroke among moderately aged and old Chinese adults over 45 years of age. METHODS: This observational cohort analysis involved 9054 participants from the Chinese Longitudinal Study of Health and Retirement and employed a standardized questionnaire administered via in-person interviews. Cox proportional hazard model, smoothed curve fitting, and threshold effect analysis were conducted for examining the potential nonlinear relationships among TyG-WC and stroke risk. RESULTS: Within an average follow-up period of six years, 463 new strokes occurred, representing 5.11% of the total number of patients. After adjusting for possible confounding factors, a nonlinear association between TyG-WC and stroke risk was identified, with a significant dose-response relationship (P = 0.023 for the log-likelihood ratio test). A turning point was identified at the TyG-WC level of 554.48, beyond that the likelihood of stroke increased markedly (HR = 1.323, 95% CI = 1.098-1.594, P = 0.003). CONCLUSION: This study revealed a specific curvilinear association with the TyG-WC score and stroke risk, identifying a key threshold value. This study focused on Chinese middle-aged and senior adults over the age of 45, emphasizing that increased stroke risk is linked to higher TyG-WC levels.

Nonlinear relationship between triglycerides and cognitive function after acute ischemic stroke among older adults.

Background: Although studies have explored the association between triglyceride levels and cognitive function after acute ischemic stroke (AIS), the results have been conflicting. Therefore, the purpose of this study was to investigate the relationship between triglyceride levels and cognitive function after AIS among older adults. Methods: This is an observational cross-sectional study. From November 2022 to June 2023, we consecutively collected patients diagnosed with AIS in China. Triglyceride levels were measured within 24 h of admission. The Mini-Mental State Examination (MMSE) was used to assess cognitive function. Nonlinear associations between triglyceride levels and cognitive function were assessed using smooth curve fitting and threshold effect analysis. Results: In this study, a total of 221 patients (mean ± SD: 70.64 ± 7.43 years) with AIS were consecutively recruited, among whom 144 (65.16%) were male. Among the 221 recruited patients, 102 (46.15%) had cognitive impairment. Triglyceride levels and cognitive impairment were found to have a nonlinear association after controlling for potential confounders, with an inflection point at 0.8 mmol/L. Below the inflection point, triglyceride levels were positively correlated with MMSE scores (ß = 14.11, 95% confidence interval [CI] = 2.33-25.89, P = 0.020). However, above the inflection point, the correlation between MMSE score and triglyceride levels was not statistically significant (ß = 1.04, 95% CI = -1.27 - 3.34, P = 0.380). Conclusion: There is a nonlinear association between triglyceride levels and cognitive function after AIS in older adults. Triglyceride was positively connected with cognitive function when it was less than 0.8 mmol/L.

Fiber-Integrated Force Sensor using 3D Printed Spring-Composed Fabry-Perot Cavities with a High Precision Down to Tens of Piconewton.

Developing microscale sensors capable of force measurements down to the scale of piconewtons is of fundamental importance for a wide range of applications. To date, advanced instrumentations such as atomic force microscopes and other specifically developed micro/nano-electromechanical systems face challenges such as high cost, complex detection systems and poor electromagnetic compatibility. Here, it presents the unprecedented design and 3D printing of general fiber-integrated force sensors using spring-composed Fabry-Perot cavities. It calibrates these microscale devices employing varied-diameter µ $\umu$ m-scale silica particles as standard weights. The force sensitivity and resolution reach values of (0.436 ± 0.007) nmnN-1 and (40.0 ± 0.7) pN, respectively, which are the best resolutions among all fiber-based nanomechanical probes so far. It also measured the non-linear airflow force distributions produced from a nozzle with an orifice of 2 µ $\umu$ m, which matches well with the full-sized simulations. With further customization of their geometries and materials, it anticipates the easy-to-use force probe can well extend to many other applications such as air/fluidic turbulences sensing, micro-manipulations, and biological sensing.

Ultrafast Förster resonance energy transfer from tyrosine to tryptophan in monellin: potential intrinsic spectroscopic ruler.

Ultrafast Förster Resonance Energy Transfer (FRET) between tyrosine (Tyr) and tryptophan (Trp) residues in the protein monellin has been investigated using picosecond and femtosecond time-resolved fluorescence spectroscopy. Decay associated spectra (DAS) and time-resolved emission spectra (TRES) taken with the different excitation wavelengths of 275, 290 and 295 nm were constructed via global analysis. At two of those three excitation loci (275 and 290 nm), earmarks of energy transfer from Tyr to Trp in monellin are seen, and particularly when the excitation is 275 nm, the energy transfer between Tyr and Trp clearly changes the signature emission DAS shape to that indicating excited state reaction (especially on the red side of fluorescence emission, near 380 nm). Those FRET signatures may overlap with the conventional signatory DAS in heterogeneous systems. When overlap and addition occur between FRET type DAS and "full positive" QSSQ (quasi-static self-quenching), mixed DAS shapes will emerge that still show "positive blue side and negative red sides", just with zero crossing shifted. In addition, excitation decay associated spectra (EDAS) taken with the different emission wavelengths of 330, 350 and 370 nm were constructed. In the study of protein dynamics, ultrafast FRET between Tyr and Trp could provide a basis for an intrinsic (non-perturbing) "spectroscopic ruler", potentially a powerful tool to detect even slight changes in protein structures.

A Scoping Review of Digital Well-Being in Early Childhood: Definitions, Measurements, Contributors, and Interventions.

Digital well-being concerns the balanced and healthy use of digital technology, and the existing studies in this area have focused on adolescents and adults. However, young children are more vulnerable to digital overuse and addiction than adults; thus, their digital well-being deserves empirical exploration. In this scoping review, we synthesized and evaluated 35 collected studies on young children's digital use and their associated well-being that were published up to October of 2022 to understand the related definitions, measurements, contributors, and interventions. The synthesis of the evidence revealed that (1) there was no consensus about the definition of the concept of digital well-being; (2) there were no effective ways of measuring young children's digital well-being; (3) both child factors (the duration and place of digital use, as well as the child's demographic characteristics) and parent factors (digital use, parental perception, and mediation) contribute to young children's well-being; and (4) there were some effective applications and digital interventions reported in the reviewed studies. This review contributes to the development of this concept by mapping the existing research on young children's digital well-being, as well as proposing a model and identifying the research gaps for future studies.

The Prevalence of Social Frailty Among Older Adults: A Systematic Review and Meta-Analysis.

OBJECTIVE: To report the overall prevalence of social frailty among older people and provide information for policymakers and authorities to use in developing policies and social care. DESIGN: A systematic review and meta-analysis. SETTING AND PARTICIPANTS: We searched 4 databases (PubMed, Embase, Web of Science, and Google Scholar) to find articles from inception to July 30, 2022. We included cross-sectional and cohort studies that provided the prevalence of social frailty among adults aged 60 years or older, in any setting. METHODS: Three researchers independently reviewed the literature and retrieved the data. A risk of bias tool was used to assess each study's quality. A random-effect meta-analysis was performed to pool the data, followed by subgroup analysis, sensitivity analysis, and meta-regression. RESULTS: From 761 records, we extracted 43 studies with 83,907 participants for meta-analysis. The pooled prevalence of social frailty in hospital settings was 47.3% (95% CI: 32.2%-62.4%); among studies in community settings, the pooled prevalence was 18.8% (95% CI: 14.9%-22.7%; P < .001). The prevalence of social frailty was higher when assessed using the Tilburg Frailty Indicator (32.3%; 95% CI: 23.1%-41.5%) than the Makizako Social Frailty Index (27.7%; 95% CI: 21.6%-33.8%) or Social Frailty Screening Index (13.4%; 95% CI: 8.4%-18.4%). Based on limited community studies in individual countries using various instruments, social frailty was lowest in China (4.9%; 95% CI: 4.2%-5.7%), followed by Spain (11.6%; 95% CI: 9.9%-13.3%), Japan (16.2%; 95% CI: 12.2%-20.3%), Korea (26.6%; 95% CI: 7.1%-46.1%), European urban centers (29.2%; 95% CI: 27.9%-30.5%), and the Netherlands (27.2%; 95% CI: 16.9%-37.5%). No other subgroup analyses showed any statistically significant prevalence difference between groups. CONCLUSION AND IMPLICATIONS: The prevalence of social frailty among older adults is high. Settings, country, and method for assessing social frailty affected the prevalence. More valid comparisons will await consensus on measurement tools and more research on geographically representative populations. Nevertheless, these results suggest that public health professionals and policymakers should seriously consider social frailty in research and program planning involving older adults.

Two-photon direct laser writing of micro Fabry-Perot cavity on single-mode fiber for refractive index sensing.

Using the two-photon polymerization (TPP) lithography, here we propose and experimentally demonstrate a fiber-tipped Fabry-Perot interferometer (FPI) for liquid refractive index (RI) measurement. To fit the aqueous environment, the FPI is designed as an open-cell microstructure consisting of well-crafted surfaces together with supporting rods, where the major spectral interference occurs between the waveguide's facet and the printed surface. Subsequently, the sensing performances of the fiber FPI are comprehensively studied under various RI as well as temperature configurations. The RI sensitivity is obtained to be ∼1058 nm/RIU with a low detection limit of 4.5× 10-6 RIU, which is comparable to that of previous reported FPIs. And the temperature cross-sensitivity reaches a value of 8.2 × 10-5 RIU/°C, indicating the good reliability for RI monitoring. Compared to other fiber FPIs, our sensor exhibits substantial advantages such as ease of fabrication, highly smooth cavity surfaces, and sufficient mechanical strength, providing a practical and competitive solution for chemical and biological sensing.

Ultrafast Förster resonance energy transfer between tyrosine and tryptophan: potential contributions to protein-water dynamics measurements.

Ultrafast Förster Resonance Energy Transfer (FRET) between Tyrosine (Tyr, Y) and Tryptophan (Trp, W) in the model peptides Trp-(Pro)n-Tyr (WPnY) has been investigated using a femtosecond up-conversion spectrophotofluorometer. The ultrafast energy transfer process (<100 ps) in short peptides (WY, WPY and WP2Y) has been resolved. In fact, this FRET rate is found to be mixed with the rates of solvent relaxation (SR), ultrafast population decay (QSSQ) and other lifetime components. To further dissect and analyze the FRET, a spectral working model is constructed, and the contribution of a FRET lifetime is separated by reconciling the shapes of decay associated spectra (DAS). Surprisingly, FRET efficiency did not decrease monotonically with the growth of the peptide chain (as expected) but increased first and then decreased. The highest FRET efficiency occurred in peptide WPY. The kinetic results have been accompanied with molecular dynamics simulations that reconcile and explain this strange phenomenon: due to the strong interaction between amino acids, the distance between the donor and receptor in peptide WPY is actually closest, resulting in the fastest FRET. In addition, the FRET lifetimes (τcal) were estimated within the molecular dynamics simulations, and they were consistent with the lifetimes (τexp) separated out by the experimental measurements and the DAS working model. This benchmark study has implications for both previous and future studies of protein ultrafast dynamics. The approach taken can be generalized for the study of proximate tyrosine and tryptophan in proteins and it suggests spectral strategies for extracting mixed rates in other complex FRET problems.

Ultrafast Excited-State Dynamics of Thiazole Orange.

Thiazole orange (TO), an asymmetric cyanine dye, has been widely used in biomolecular detection and imaging of DNA/ RNA in gels, due to its unique fluorogenic behavior: fluorescence of free dye in aqueous solution is very weak but emission can be significantly enhanced in nucleic-acid-bound dye. Herein we describe the ultrafast excited-state dynamics of free TO in aqueous solution by exploiting both a femtosecond upconversion spectrophotofluorometer and a picosecond time-correlated single-photon counting (TCSPC) apparatus. For the first time, the fluorescence lifetime of TO monomer in water was found to be ∼1 ps, mixed with concurrent solvent relaxation (which was confirmed by the experimental results of TO in DMSO). Even at moderate concentration, this lifetime has an amplitude (a measure of molecular fraction) that significantly dominates other lifetimes, and this is the origin of weak steady state fluorescence of free TO in water. We also found a novel slower decay component around 34 ps. Interestingly and in addition, the lifetime component on the 30-40 ps timescale was also found in TO-γ-Cyclodextrin (CD) complexes. The fraction of this component increased with the addition of γ-CD. Cyclodextrin has been reported to promote the aggregation of TO. Thus, although a very coincidental match of this time constant by one for a torsional process within the cavity can not be ruled out, we ascribe the shared 30-40 ps component to the lifetime of a highly quenched TO dimer experiencing intra-and inter-molecular rearrangement.

Excited State Dynamics of Methylated Guanosine Derivatives Revealed by Femtosecond Time-resolved Spectroscopy.

Methylated DNA/RNA nucleobases are important epigenetic marks in living species and play an important role for targeted therapies. Moreover, methylation could bring significant changes to the photo-stability of nucleic acid, leading these sites become mutational hotspots for disease such as skin cancer. While a number of studies have demonstrated the relationship between excited state dynamics and the biological function of methylated cytosine in DNA, investigations aimed at unraveling the excited state dynamics of methylated guanosine in RNA have been largely overlooked. In this work, influence of methylation on the excited state dynamics of guanosine is studied by using femtosecond time-resolved spectroscopy. Our results suggest that the effect of methyl substitution on the photophysical properties of guanosine is position sensitive. N1-methylguanosine shows very similar excited state dynamics as that in guanosine, while almost one order of magnitude longer lifetime of the La state is observed in N2, N2-dimethylguanosine. Notably, N7-methylation can lead to a new minimum on the La state and the excited state lifetime is two orders of magnitude longer than that of guanosine. These findings not only help understanding excited state dynamics of methylated guanosines, but also lay the foundation for further studying DNA/RNA strands incorporated with these bases.

Atomically dispersed Fe-N-C catalyst displaying ultra-high stability and recyclability for efficient electroreduction of CO2 to CO.

To avoid the agglomeration of iron NPs and improve the dispersion of Fe SAs, we employed a mixed-ligand strategy to regulate the iron content in PCN-224(ZnxFey) and PCN-222(ZnxFey). Thanks to the sublimation of Zn and the Kirkendall effect, uniform dispersions of Fe SAs with 1.04-1.06 wt% were obtained in the pyrolysis products Zn0.5Fe0.5-N-C-224 and Zn0.5Fe0.5-N-C-222 with excellent CO2 → CO activity, super-stability, and recyclability.

Attention-Guided Network with Densely Connected Convolution for Skin Lesion Segmentation.

The automatic segmentation of skin lesions is considered to be a key step in the diagnosis and treatment of skin lesions, which is essential to improve the survival rate of patients. However, due to the low contrast, the texture and boundary are difficult to distinguish, which makes the accurate segmentation of skin lesions challenging. To cope with these challenges, this paper proposes an attention-guided network with densely connected convolution for skin lesion segmentation, called CSAG and DCCNet. In the last step of the encoding path, the model uses densely connected convolution to replace the ordinary convolutional layer. A novel attention-oriented filter module called Channel Spatial Fast Attention-guided Filter (CSFAG for short) was designed and embedded in the skip connection of the CSAG and DCCNet. On the ISIC-2017 data set, a large number of ablation experiments have verified the superiority and robustness of the CSFAG module and Densely Connected Convolution. The segmentation performance of CSAG and DCCNet is compared with other latest algorithms, and very competitive results have been achieved in all indicators. The robustness and cross-data set performance of our method was tested on another publicly available data set PH2, further verifying the effectiveness of the model.

Ultrafast Fluorescence Spectroscopy via Upconversion and Its Applications in Biophysics.

In this review, the experimental set-up and functional characteristics of single-wavelength and broad-band femtosecond upconversion spectrophotofluorometers developed in our laboratory are described. We discuss applications of this technique to biophysical problems, such as ultrafast fluorescence quenching and solvation dynamics of tryptophan, peptides, proteins, reduced nicotinamide adenine dinucleotide (NADH), and nucleic acids. In the tryptophan dynamics field, especially for proteins, two types of solvation dynamics on different time scales have been well explored: ~1 ps for bulk water, and tens of picoseconds for "biological water", a term that combines effects of water and macromolecule dynamics. In addition, some proteins also show quasi-static self-quenching (QSSQ) phenomena. Interestingly, in our more recent work, we also find that similar mixtures of quenching and solvation dynamics occur for the metabolic cofactor NADH. In this review, we add a brief overview of the emerging development of fluorescent RNA aptamers and their potential application to live cell imaging, while noting how ultrafast measurement may speed their optimization.

Revealing the structure-activity relationship of two Cu-porphyrin-based metal-organic frameworks for the electrochemical CO2-to-HCOOH transformation.

The eCO2RR activity is correlated to the internal structural character of the catalyst. We employed two types of structural models of porphyrin-based MOFs of PCN-222(Cu) and PCN-224(Cu) into heterogeneous catalysis to illustrate the effect of structural factors on the eCO2RR performance. The composite catalyst PCN-222(Cu)/C displays better activity and selectivity (η = 450 mV, FEHCOOH = 44.3%, j = 3.2 mA cm-2) than PCN-224(Cu)/C (η = 450 mV, FEHCOOH = 34.1%, j = 2.4 mA cm-2) for the CO2 reduction to HCOOH in the range of -0.7--0.9 V (vs. RHE) due to its higher BET surface area, CO2 uptake, and a larger pore diameter. It is interesting that PCN-224(Cu)/C displays better performance in the range of -0.4--0.6 V (vs. RHE) due to its greater heat of adsorption, Qst and a higher affinity for CO2 molecule, which could promote the capture of CO2 onto the exposed active sites. As a result, PCN-224(Cu)/C exhibits better stability for the long-term electrolysis.

Young children's online learning during COVID-19 pandemic: Chinese parents' beliefs and attitudes.

Online learning has been widely promoted to replace traditional face-to-face learning during the COVID-19 pandemic to maintain young children's learning and play at home. This study surveyed 3275 Chinese parents' beliefs and attitudes around young children's online learning during the lockdown of the COVID-19 pandemic. Most parents (92.7%) in the study reported that their children had online learning experiences during the pandemic, and many (84.6%) spent less than a half-hour each time. The parents generally had negative beliefs about the values and benefits of online learning and preferred traditional learning in early childhood settings. They tended to resist and even reject online learning for three key reasons: the shortcomings of online learning, young children's inadequate self-regulation, and their lack of time and professional knowledge in supporting children's online learning. Also, the hardship caused by the COVID-19 pandemic has made them suffering, thus more resistant to online learning at home. The results suggested that the implementation of online learning during the pandemic has been problematic and challenging for families. The Chinese parents were neither trained nor ready to embrace online learning. The paper concluded with implications for policymakers and teacher education.

Ultrafast spectroscopy of biliverdin dimethyl ester in solution: pathways of excited-state depopulation.

Biliverdin is a bile pigment that has a very low fluorescence quantum yield in solution, but serves as a chromophore in far-red fluorescent proteins being developed for bio-imaging. In this work, excited-state dynamics of biliverdin dimethyl ether (BVE) in solvents were investigated using femtosecond (fs) and picosecond (ps) time-resolved absorption and fluorescence spectroscopy. This study is the first fs timescale investigation of BVE in solvents, and therefore revealed numerous dynamics that were not resolved in previous, 200 ps time resolution measurements. Viscosity- and isotope-dependent experiments were performed to identify the contributions of isomerization and proton transfer to the excited-state dynamics. In aprotic solvents, a ∼2 ps non-radiative decay accounts for 95% of the excited-state population loss. In addition, a minor ∼30 ps emissive decay pathway is likely associated with an incomplete isomerization process around the C15[double bond, length as m-dash]C16 double bond that results in a flip of the D-ring. In protic solvents, the dynamics are more complex due to hydrogen bond interactions between solute and solvent. In this case, the ∼2 ps decay pathway is a minor channel (15%), whereas ∼70% of the excited-state population decays through an 800 fs emissive pathway. The ∼30 ps timescale associated with isomerization is also observed in protic solvents. The most significant difference in protic solvents is the presence of a >300 ps timescale in which BVE can decay through an emissive state, in parallel with excited-state proton transfer to the solvent. Interestingly, a small fraction of a luminous species, which we designate lumin-BVE (LBVE), is present in protic solvents.

Dehydrogenase Binding Sites Abolish the "Dark" Fraction of NADH: Implication for Metabolic Sensing via FLIM.

The fluorescence of dinucleotide NADH has been exploited for decades to determine the redox state of cells and tissues in vivo and in vitro. Particularly, nanosecond (ns) fluorescence lifetime imaging microscopy (FLIM) of NADH (in free vs bound forms) has recently offered a label-free readout of mitochondrial function and allowed the different "pools" of NADH to be distinguished in living cells. In this study, the ultrafast fluorescence dynamics of NADH-dehydrogenase (MDH/LDH) complexes have been investigated by using both a femtosecond (fs) upconversion spectrophotofluorometer and a picosecond (ps) time-correlated single photon counting (TCSPC) apparatus. With these enhanced time-resolved tools, a few-picosecond decay process with a signatory spectrum was indeed found for bound NADH, and it can best be ascribed to the solvent relaxation originating in "bulk water". However, it is quite unlike our previously discovered ultrafast "dark" component (∼26 ps) that is prominent in free NADH (Chemical Physics Letters 2019, 726, 18-21). For these two critical protein-bound NADH exemplars, the decay transients lack the ultrafast quenching that creates the "dark" subpopulation of free NADH. Therefore, we infer that the apparent ratio of free to bound NADH recovered by ordinary (>50 ps) FLIM methods may be low, since the "dark" molecule subpopulation (lifetime too short for conventional FLIM), which effectively hides about a quarter of free molecules, is not present in the dehydrogenase-bound state.

Femtosecond Fluorescence Spectra of NADH in Solution: Ultrafast Solvation Dynamics.

The ultrafast solvation dynamics of reduced nicotinamide adenine dinucleotide (NADH) free in solution has been investigated, using both a femtosecond upconversion spectrophotofluorometer and a picosecond time-correlated single-photon counting (TCSPC) apparatus. The familiar time constant of solvent relaxation originating in "bulk water" was found to be ∼1.4 ps, revealing ultrafast solvent reorientation upon excitation. We also found a slower spectral relaxation process with an apparent time of 27 ps, suggesting there could either be dissociable "biological water" hydration sites on the surface of NADH or internal dielectric rearrangements of the flexible solvated molecule on that timescale. In contrast, the femtosecond fluorescence anisotropy measurement revealed that rotational diffusion happened on two different timescales (3.6 ps (local) and 141 ps (tumbling)); thus, any dielectric rearrangement scenario for the 27 ps relaxation must occur without significant chromophore oscillator rotation. The coexistence of quasi-static self quenching (QSSQ) with the slower relaxation is also discussed.