Quantification of the surface density of a fluorescent label with the optical microscope

Proteomic Analysis of Biomaterial Surfaces after Contacting with Body Fluids by MALDI-ToF Mass Spectroscopy

We developed a method to identify proteins adsorbed on solid surfaces from a solution containing a complex mixture of proteins by using Matrix-Assisted Laser Desorption/Ionization-Time of Flight mass (MALDI-ToF mass) spectroscopy. In the method, we performed all procedures of peptide mass fingerprint method including denaturation, reduction, alkylation, digestion, and spotting of matrix on substrates. The method enabled us to avoid artifacts of pipetting that could induce changes in the composition. We also developed an algorithm to identify the adsorbed proteins. In this work, we demonstrate the identification of proteins adsorbed on self-assembled monolayers (SAMs). Our results show that the composition of proteins on the SAMs critically depends on the terminal groups of the molecules constituting the SAMs, indicating that the competitive adsorption of protein molecules is largely affected by protein-surface interaction. The method introduced here can provide vital information to clarify the mechanism underlying the responses of cells and tissues to biomaterials.

Intensity calibration and flat-field correction for fluorescence microscopes

Standardization in fluorescence microscopy involves calibration of intensity in reproducible units and correction for spatial nonuniformity of illumination (flat-field or shading correction). Both goals can be achieved using concentrated solutions of fluorescent dyes. When a drop of a highly concentrated fluorescent dye is placed between a slide and a coverslip it produces a spatially uniform field, resistant to photobleaching and with reproducible quantum yield; it can be used as a brightness standard for wide-field and confocal microscopes. For wide-field microscopes, calibration can be further extended to absolute molecular units. This can be done by imaging a solution of known concentration and known depth; the latter can be prepared by placing a small spherical lens in a diluted solution of the same fluorophore that is used in the biological specimen.

Programming and use of Parylene C fluorescence as a quantitative on-chip reference

A large number of lab-on-a-chip applications use fluorescence for quantifying biological entities.

Cell cycle dependent TN-C promoter activity determined by live cell imaging

The extracellular matrix protein tenascin-C plays a critical role in development, wound healing, and cancer progression, but how it is controlled and how it exerts its physiological responses remain unclear. By quantifying the behavior of live cells with phase contrast and fluorescence microscopy, the dynamic regulation of TN-C promoter activity is examined. We employ an NIH 3T3 cell line stably transfected with the TN-C promoter ligated to the gene sequence for destabilized green fluorescent protein (GFP). Fully automated image analysis routines, validated by comparison with data derived from manual segmentation and tracking of single cells, are used to quantify changes in the cellular GFP in hundreds of individual cells throughout their cell cycle during live cell imaging experiments lasting 62 h. We find that individual cells vary substantially in their expression patterns over the cell cycle, but that on average TN-C promoter activity increases during the last 40% of the cell cycle. We also find that the increase in promoter activity is proportional to the activity earlier in the cell cycle. This work illustrates the application of live cell microscopy and automated image analysis of a promoter-driven GFP reporter cell line to identify subtle gene regulatory mechanisms that are difficult to uncover using population averaged measurements.

Fluorescent labeling of chitosan for use in non-invasive monitoring of degradation in tissue engineering

The establishment of non-invasive analytical tools for assessing the in-situ use of biomaterials for surgical implants or scaffolds in tissue engineering and polymer-based therapies is fundamental. This study established a method for fluorescent tracking of the degradation of a chitosan membrane scaffold for use in vitro in bioreactors and ultimately in vivo. The basis of this tracking system is a fluorescence emitting biomaterial obtained by covalent binding of the fluorophore tetramethylrhodamine isothiocyanate (TRITC) onto the backbone of chitosan. Using confocal microscopy, this study quantitated the reductions in fluorescence intensity of the membrane and correlated these decreases with weight loss during polymer breakdown, thereby providing a technique for non-destructively assessing the extent of degradation of chitosan materials over time in vitro. Using multispectral imaging in a mouse model, the study assessed the degradation profile of the fluorophore-labeled biomaterial in vivo in real time and identified the dispersing pathway of the chitosan membrane degradation products in vivo. The results revealed that TRITC conjugated chitosan was biocompatible and supported bone cell growth. The changes in fluorescence intensity correlated well with weight loss up to 16 weeks of in vitro culture and could be monitored over two weeks in vivo.

Preparation of protein- and cell-resistant surfaces by hyperthermal hydrogen induced cross-linking of poly(ethylene oxide)

The functionalization of surfaces with poly(ethylene oxide) (PEO) is an effective means of imparting resistance to the adsorption of proteins and the attachment and growth of cells, properties that are critical for many biomedical applications. In this work, a new hyperthermal hydrogen induced cross-linking (HHIC) method was explored as a simple one-step approach for attaching PEO to surfaces through the selective cleavage of C-H bonds and subsequent cross-linking of the resulting carbon radicals. In order to study the effects of the process on the polymer, PEO-coated silicon wafers were prepared and the effects of different treatment times were investigated. Subsequently, using an optimized treatment time and a modified butyl polymer with increased affinity for PEO, the technique was applied to butyl rubber surfaces. All of the treated surfaces exhibited significantly reduced protein adsorption and cell growth relative to control surfaces and compared favorably with surfaces that were functionalized with PEO using conventional chemical methods. Thus HHIC is a simple and effective means of attaching PEO to non-functional polymer surfaces.

Dynamics and mechanisms of quantum dot nanoparticle cellular uptake

BACKGROUND

The rapid growth of the nanotechnology industry and the wide application of various nanomaterials have raised concerns over their impact on the environment and human health. Yet little is known about the mechanism of cellular uptake and cytotoxicity of nanoparticles. An array of nanomaterials has recently been introduced into cancer research promising for remarkable improvements in diagnosis and treatment of the disease. Among them, quantum dots (QDs) distinguish themselves in offering many intrinsic photophysical properties that are desirable for targeted imaging and drug delivery.

RESULTS

We explored the kinetics and mechanism of cellular uptake of QDs with different surface coatings in two human mammary cells. Using fluorescence microscopy and laser scanning cytometry (LSC), we found that both MCF-7 and MCF-10A cells internalized large amount of QD655-COOH, but the percentage of endocytosing cells is slightly higher in MCF-7 cell line than in MCF-10A cell line. Live cell fluorescent imaging showed that QD cellular uptake increases with time over 40 h of incubation. Staining cells with dyes specific to various intracellular organelles indicated that QDs were localized in lysosomes. Transmission electron microscopy (TEM) images suggested a potential pathway for QD cellular uptake mechanism involving three major stages: endocytosis, sequestration in early endosomes, and translocation to later endosomes or lysosomes. No cytotoxicity was observed in cells incubated with 0.8 nM of QDs for a period of 72 h.

CONCLUSIONS

The findings presented here provide information on the mechanism of QD endocytosis that could be exploited to reduce non-specific targeting, thereby improving specific targeting of QDs in cancer diagnosis and treatment applications. These findings are also important in understanding the cytotoxicity of nanomaterials and in emphasizing the importance of strict environmental control of nanoparticles.

Measurement of wheat germ agglutinin binding with a fluorescence microscope

Signal intensity in fluorescence microscopy is often measured relative to arbitrary standards. We propose a calibration method based on a solution of the same fluorophore, whose binding to cells needs to be quantified. The method utilizes the low sensitivity of intensity to the object distance in wide-field imaging of uniform materials. Liquid layers of slowly varying depth were prepared by immersing a spherical lens into a drop of a fluorophore placed on a slide. Flatfield-corrected images of the contact and surrounding areas showed linear dependence of the gray level on the depth of fluorescent liquid. This allowed conversion of the measured intensity into the number of molecules per unit area. The method was applied to different cell types stained by WGA-Alexa 488 and WGA-TRITC. Consistent results were obtained by comparing microscopy with flow cytometry, comparing imaging through different objectives and comparing different WGA conjugates. Reproducibility of calibration was within 97% when low magnification was used. Fluorescence of free and bound WGA was found to be different, however, and therefore precise measurement of the number of cell-bound molecules was problematic in this particular system. We conclude that the method achieves reliable measurement of cellular staining in the units of soluble fluorophore. For probes whose fluorescent properties are unaffected by binding, quantification of staining in true molecular units should be possible.

Effect of functional end groups of silane self-assembled monolayer surfaces on apatite formation, fibronectin adsorption and osteoblast cell function

Bioactive glass (BG) can directly bond to living bone without fibrous tissue encapsulation. Key mechanistic steps of BG's activity are attributed to calcium phosphate formation, surface hydroxylation and fibronectin (FN) adsorption. In the present study, self-assembled monolayers (SAMs) of alkanesilanes with different surface chemistry (OH, NH(2) and COOH) were used as a model system to mimic BG's surface activity. Calcium phosphate (Ca-P) was formed on SAMs by immersion in a solution that simulates the electrolyte content of physiological fluids. FN adsorption kinetics and monolayer coverage was determined on SAMs with or without Ca-P coating. The surface roughness was also examined on these substrates before and after FN adsorption. The effects of FN-adsorbed, Ca-P-coated SAMs on the function of MC3T3-E1 were evaluated by cell growth, expression of alkaline phosphatase activity and actin cytoskeleton formation. We demonstrate that, although the FN monolayer coverage and the root mean square (rms) roughness are similar on --OH and --COOH terminated SAMs with or without Ca-P coating, higher levels of ALP activity, more actin cytoskeleton formation and more cell growth are obtained on --OH- and --COOH-terminated SAMs with Ca-P coating. In addition, although the FN monolayer coverage is higher on Ca-P-coated --NH(2)-terminated SAMs and SiO(x) surfaces, higher levels of ALP activity and more cell growth are obtained on Ca-P-coated --OH- and --COOH-terminated SAMs. Thus, with the same Ca-P coatings, different surface functional groups have different effects on the function of osteoblastic cells. These findings represent new insights into the mechanism of bioactivity of BG and thereby may lead to designing superior constructs for bone grafting.

Surface plasmon resonance imaging of cells and surface-associated fibronectin

Background

A critical challenge in cell biology is quantifying the interactions of cells with their extracellular matrix (ECM) environment and the active remodeling by cells of their ECM. Fluorescence microscopy is a commonly employed technique for examining cell-matrix interactions. A label-free imaging method would provide an alternative that would eliminate the requirement of transfected cells and modified biological molecules, and if collected nondestructively, would allow long term observation and analysis of live cells.

Results

Using surface plasmon resonance imaging (SPRI), the deposition of protein by vascular smooth muscle cells (vSMC) cultured on fibronectin was quantified as a function of cell density and distance from the cell periphery. We observed that as much as 120 ng/cm² of protein was deposited by cells in 24 h.

Conclusion

SPRI is a real-time, low-light-level, label-free imaging technique that allows the simultaneous observation and quantification of protein layers and cellular features. This technique is compatible with live cells such that it is possible to monitor cellular modifications to the extracellular matrix in real-time.

A functionalized poly(ethylene glycol)-based bioassay surface chemistry that facilitates bio-immobilization and inhibits non-specific protein, bacterial, and mammalian cell adhesion

This paper describes a new bioassay surface chemistry that effectively inhibits non-specific biomolecular and cell binding interactions, while providing a capacity for specific immobilization of desired biomolecules. Poly(ethylene glycol) (PEG) as the primary component in nonfouling film chemistry is well-established, but the multicomponent formulation described here is unique in that it (1) is applied in a single, reproducible, solution-based coating step; (2) can be applied to diverse substrate materials without the use of special primers; and (3) is readily functionalized to provide specific attachment chemistries. Surface analysis data are presented, detailing surface roughness, polymer film thickness, and film chemistry. Protein non-specific binding assays demonstrate significant inhibition of serum, fibrinogen, and lysozyme adsorption to coated glass, indium tin oxide, and tissue culture polystyrene dishes. Inhibition of S. aureus and K. pneumoniae microbial adhesion in a microfluidic flow cell, and inhibition of fibroblast cell adhesion from serum-based cell culture is shown. Effective functionalization of the coating is demonstrated by directing fibroblast adhesion to polymer surfaces activated with an RGD peptide. Batch-to-batch reproducibility data are included. The in situ cross-linked PEG-based coating chemistry is unique in its formulation, and its surface properties are attractive for a broad range of in vitro bioassay applications.

Measurement of the absorption of concentrated dyes and their use for quantitative imaging of surface topography

We propose a method to image the surface topography of transparent objects. The space between the object and the opposite closely positioned surface (such as a cover glass or a slide) is filled with a strongly absorbing dye. The contrast is generated by recording a transmission image at a wavelength where the dye absorbs. Since the transmitted intensity depends on the depth of the dye layer, it carries information about the relief of the tested surface. With sufficiently concentrated dyes, nanometre unevenness of a surface can be detected. By using less-concentrated solutions, it is possible to image and measure larger objects, such as biological cells. At the present stage, biological applications of the method are only semi-quantitative, but the method still provides detailed information about cell shapes that is not readily obtainable with other imaging techniques. Conversion of the image grey scale into the units of vertical distance requires knowledge of the absorption coefficient of the dye. The same method that is used for imaging can be adapted to measure the absorption coefficient of concentrated dyes. The solution to be analyzed is placed between a glass slide and a spherical lens of known radius. The absorption coefficient is determined from attenuation of transmitted intensity as a function of the distance to the centre. At the same time, the interference pattern in the reflected image allows measurement of the refractive index of the dye.

Relating material surface heterogeneity to protein adsorption: the effect of annealing of micro-contact-printed OTS patterns

We have investigated the influence of micrometer- and sub-micrometer-scale surface heterogeneities in patterned octadecyltrichlorosilane (OTS) films on human serum albumin (HSA) adsorption and its spatial distribution. 5-μm-wide OTS patterns were created on glass substrates by micro-contact printing and in some instances subsequent annealing was used to alter OTS molecule distribution within the patterns. Scanning force microscopy (SFM), advancing water contact angles and water vapor condensation figures were used to characterize the OTS films and to assess the nature of the heterogeneities within the various surface areas. High-resolution fluorescence microscopy was used to record images of fluorescently labeled albumin on OTS patterned films and fluorescence intensity was quantified and converted into the adsorbed amount. Adsorbed albumin was also characterized through SFM measurements. Combined SFM topography and lateral force microscopy (LFM) imaging revealed that micro-contact printing of OTS onto glass both replicated the stamp pattern and created small islands within the non-stamped regions between the patterns. The OTS coverage within stamped regions was not fully continuous but improved with subsequent annealing. Annealing also resulted in OTS island growth within the non-stamped regions and decreased average wettability on both the stamped and non-stamped areas. The extent of albumin adsorption was not proportional to OTS coverage, but correlated with the sub-μm distribution of OTS chains. We inferred that the surface distribution of ligands such as OTS on a sub-μm length scale determines the nature of albumin adsorption and its kinetics.

Automated live cell imaging of green fluorescent protein degradation in individual fibroblasts

To accurately interpret the data from fluorescent proteins as reporters of gene activation within living cells, it is important to understand the kinetics of the degradation of the reporter proteins. We examined the degradation kinetics over a large number (>1,000) of single, living cells from a clonal population of NIH3T3 fibroblasts that were stably transfected with a destabilized, enhanced green fluorescent protein (eGFP) reporter driven by the tenascin-C promoter. Data collection and quantification of the fluorescence protein within a statistically significant number of individual cells over long times (14 h) by automated microscopy was facilitated by culturing cells on micropatterned arrays that confined their migration and allowed them to be segmented using phase contrast images. To measure GFP degradation rates unambiguously, protein synthesis was inhibited with cycloheximide. Results from automated live cell microscopy and image analysis indicated a wide range of cell-to-cell variability in the GFP fluorescence within individual cells. Degradation for this reporter was analyzed as a first order rate process with a degradation half-life of 2.8 h. We found that GFP degradation rates were independent of the initial intensity of GFP fluorescence within cells. This result indicates that higher GFP abundance in some cells is likely due to higher rates of gene expression, because it is not due to systematically lower rates of protein degradation. The approach described in this study will assist the quantification and understanding of gene activity within live cells using fluorescent protein reporters.

Fluorescent resonance energy transfer: A tool for probing molecular cell-biomaterial interactions in three dimensions

The current paradigm in designing biomaterials is to optimize material chemical and physical parameters based on correlations between these parameters and downstream biological responses, whether in vitro or in vivo. Extensive developments in molecular design of biomaterials have facilitated identification of several biophysical and biochemical variables (e.g. adhesion peptide density, substrate elastic modulus) as being critical to cell response. However, these empirical observations do not indicate whether different parameters elicit cell responses by modulating redundant variables of the cell-material interface (e.g. number of cell-material bonds, cell-matrix mechanics). Recently, fluorescence resonance energy transfer (FRET) has been applied to quantitatively analyze parameters of the cell-material interface for both two- and three-dimensional adhesion substrates. Tools based on FRET have been utilized to quantify several parameters of the cell-material interface relevant to cell response, including molecular changes in matrix proteins induced by interactions both with surfaces and cells, the number of bonds between integrins and their adhesion ligands, and changes in the crosslink density of hydrogel synthetic extracellular matrix analogs. As such techniques allow both dynamic and 3-D analyses they will be useful to quantitatively relate downstream cellular responses (e.g. gene expression) to the composition of this interface. Such understanding will allow bioengineers to fully exploit the potential of biomaterials engineered on the molecular scale, by optimizing material chemical and physical properties to a measurable set of interfacial parameters known to elicit a predictable response in a specific cell population. This will facilitate the rational design of complex, multi-functional biomaterials used as model systems for studying diseases or for clinical applications.

Ligand density characterization of peptide-modified biomaterials

A simple fluorescence based characterization method was developed to assess ligand density on peptide-modified biomaterials. The method exploits the exquisite sensitivity of proteolysis for the purpose of liberating a fluorescently labeled probe fragment from an immobilized peptide. The released fragment can then be detected in solution using high-throughput fluorometry. In silico screening tools identified the enzyme chymotrypsin as a promising candidate for releasing a detectable probe fragment from the fluorescently labeled peptide, Ac-CGGNGEPRGDTYRAYK(FITC)GG-NH(2). After chymotrypsin digestion of the peptide in solution was first characterized using mass spectrometry and HPLC, a basic enzyme mediated release protocol was developed and implemented to generate peptide-binding isotherms on various peptide-modified biomaterials. The new method is sensitive, has good signal-to-noise ratio (S/N), and is easily standardized. Furthermore, the technique can be applied independent of material chemistry and geometry, making it a suitable alternative to radiolabeling for a wide range of biomaterial applications.

Analysis of interpenetrating polymer networks via quartz crystal microbalance with dissipation monitoring

A quartz crystal microbalance with dissipation monitoring (QCM-D) was used to assess the physical properties of interpenetrating polymer networks (IPNs) through swelling experiments in ambient humidity and in phosphate-buffered saline (PBS), pH 7.4. The IPNs, based on acrylamide (AAm) and poly(ethylene glycol) (pEG), swell from thin, rigid films when dry (16.7 +/- 5.2 nm on Si/SiO(2)) to expanded, viscoelastic films when hydrated (107 +/- 24.2 nm on Si/SiO2). The dry IPNs could be analyzed using the Sauerbrey relationship, but for the hydrated films it was necessary to interpret QCM-D data with a Kelvin-Voigt viscoelastic model. A complex modulus |G| of 116 +/- 38.1 kPa for the swollen IPN surface on Si/SiO2 was defined by the model. The QCM-D was also employed to quantify the adsorption of human fibrinogen, a protein important in thrombus formation, onto the IPNs. Fibrinogen adsorption studies demonstrated the sensitivity of the QCM-D, as well as confirmed the nonfouling nature of the IPN surface, where less than 5 ng/cm2 of fibrinogen was adsorbed.

Fibronectin adsorption on surface-activated poly(dimethylsiloxane) and its effect on cellular function

This article reports that surface modification of poly(dimethylsiloxane) (PDMS) influences fibronectin (Fn) adsorption and enhances cell attachment. Controlled adsorption of Fn on chemically activated polymer substrates is known to influence cellular function. Thin films of PDMS were spun cast on silicon wafers to obtain homogeneous and molecularly smooth surfaces. The films were made hydrophilic by exposure to ultraviolet ozone activation (PDMS*). The films then were characterized by contact angle goniometry, ellipsometry, atomic force microscopy (AFM), Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy. Contact angle measurements indicated higher hydrophobicity of the nonactivated PDMS substrates than PDMS*. AFM scans of the substrates indicated higher surface roughness of PDMS* (Ra = 0.55 nm) than PDMS (Ra = 0.25 nm). Although Fn surface density (Gamma) was slightly higher on PDMS than on PDMS*, due to hydrophobic interactions between substrate and Fn, cell function was greatly enhanced on the Fn-coated PDMS* (PDMS*-Fn) than on PDMS (PDMS-Fn). Higher attachment of MC3T3-E1 osteoblast-like cells was observed on PDMS*-Fn than on PDMS-Fn. Moreover, cell spreading and cytoskeleton organization after 72 h was clearly favored on the Fn-coated PDMS* surfaces.

Analysis of immobilizedL-cysteine on polymers

Recently, we reported that L-cysteine attached to polymeric biomaterials, without prior nitrosation, enhances the hemocompatibility of biomaterials via exploiting endogenous nitric oxide (NO). As part of the polymer optimization process to further enhance platelet inhibition, a kinetic model is being developed to predict the release rate of NO. A key model parameter is the immobilized concentration of L-cysteine. This article demonstrates how several chemiluminescence-based assays, previously utilized for measuring thiols in solution, were successfully adapted to quantify immobilized L-cysteine. The assays showed that the immobilized L-cysteine on the modified PET sample is within the range of 4.1 to 6.5 nmol/cm(2). An advantage of using the more successful chemiluminescence-based assay is that it can accurately measure molar concentrations of any thiol-containing compound with a detection limit in the pmol range. The major disadvantage is that L-cysteine must first be broken off of the polymer and released into solution prior to measurement, therefore leaving the sample unable to be reused. Other thiol-measuring techniques, such as fluorescence microscopy and X-ray photoelectron spectroscopy (XPS), were used to provide qualitative and semiquantitative analysis to substantiate the polymer development.

Effects of contrast media on erythrocyte aggregation during sedimentation

To evaluate the effects of contrast media (CMs) on erythrocyte aggregation, we measured the erythrocyte sedimentation with Westergren method at 25 degrees C. CMs were diatrizoate (Urografin 76%) for ionic CM and iopamidol (Iopamiron 370) for nonionic CM. Swine red blood cells (RBCs) were suspended in autologous plasma containing diatrizoate (URO), iopamidol (IOP), and saline (SAL) at 6.7% w/w, as well as in plasma alone (PLA), at 40% of the hematocrit. Sigmoid sedimentation curves were fitted to the Puccini et al. (1977) equation, and the average number of RBCs per aggregate m was calculated by Stokes' law against the time t. According to the Murata-Secomb (1988) theory we estimated the collision rate K between two aggregates from dm/dt in the stationary phase during sedimentation. Corresponding to the maximal ESR, the dm/dt (in cells/s) was 0.52 in PLA, 0.09 in SAL, 0.06 in URO and 0.03 in IOP, so that K also decreased in proportion to dm/dt from 145 fL/s in PLA to 8 fL/s in IOP. Both the ionic and nonionic CMs tend to inhibit the RBC aggregation more than that in SAL; the latter iopamidol appears to be inhibitory more than the former diatrizoate in autologous plasma.

Quantitative analysis of binary adsorbed protein films by time of flight secondary ion mass spectrometry

Time of flight secondary ion mass spectrometry (ToF-SIMS) is an ideal technique for the analysis of adsorbed protein films because of its surface sensitivity and chemical specificity. In this study, we examined ToF-SIMS with the multivariate calibration method partial least squares regression (PLSR) for the determination of the relative abundance of the components in binary protein films adsorbed onto mica, PTFE, and heptyl amine plasma polymer substrates. These results have been compared with independently measured 125I-radiolabeled protein adsorption experiments. By applying PLSR to the ToF-SIMS data, the relative abundance of the components in the binary adsorbed protein films was quantified, and the agreement between the ToF-SIMS and 125I-radiolabeling data was measured by the root mean square prediction error (RMSPE). Differences in protein quantification by PLSR and 125I-radiolabeling ranged from 5 to 25 mass % RMSPE and were highly dependent on the structure of the adsorbed protein film, the substrate surface chemistry and morphology, and the number of latent variables retained in the PLSR model. The limit of detection for the minor component in the adsorbed protein film was found to be approximately 10 mass %. This study demonstrates that the combination of ToF-SIMS and multivariate calibration provide complementary information to 125I-radiolabeling about the composition and structure of binary adsorbed protein films.

A standard for calibration and shading correction of a fluorescence microscope

BACKGROUND

Numerous applications of fluorescence microscopy require quantitation of signal intensity in reproducible units. Two problems must be overcome to achieve this goal. First, due to various instrumental factors, the same sample imaged on two microscopes or even on the same microscope at different times may produce highly divergent readings. Second, because of shading, some areas within the same field may appear brighter than others despite the same amount of fluorophore. The first type of variability requires calibration using a sample of reproducible fluorescence yield; to correct for shading, a uniform fluorescent field is needed.

METHODS

Standard slides were prepared by placing several microliters of 10%-50% w/v fluorescein or rhodamine between a coverglass and a slide. They were used to perform shading correction and normalization under a variety of imaging conditions.

RESULTS

Concentrated fluorophores produced a uniform fluorescent field of moderate and reproducible brightness. By expressing the staining of a biological object in the units of standard slides, identical results were obtained irrespective of the imaging conditions or the microscope used. We compared shading correction based on concentrated fluorescein with two other standards. Concentrated fluorescein resulted in the best equalization of the field.

CONCLUSIONS

Standardization of fluorescent images can be achieved by normalizing them to the image of a concentrated solution of a fluorophore. Due to its simplicity and efficiency, this method can be used in clinical analysis as well as in routine laboratory practice.