The fluorescence of Lignum nephriticum: a flash back to the past,
and a simple demonstration of natural substance fluorescence.
Mark Muyskens, Department of Chemistry and Biochemistry, Calvin College, Grand Rapids,
Michigan 49546 (email: muym@calvin.edu)
Manuscript accepted for publication in the Journal of Chemical Education
Abstract
This article describes a simple but visually striking demonstration of fluorescence from the
aqueous extract of the tropical hardwood Pterocarpus indicus. It illustrates the first recorded
observation of fluorescence, noted over 400 years ago when the wood was known as Lignum
nephriticum. The fact that the strong, blue fluorescence is dramatically pH dependent provides
an interesting dimension to the demonstration. A yellow filter allows a simple demonstration of
the Stokes shift. Highlighting the historical significance of Lignum nephriticum and discussing
some of the relevant studies suggesting the chemical origin of the fluorescence provides a
context for this demonstration. A companion JCE Classroom Activity provides a hands-on
activity illustrating the fluorescence.
Keywords
Audience: Elementary / Middle School, High School / Introductory Chemistry, First-Year
Undergraduate / General, Second-Year Undergraduate, Upper-Division Undergraduate.
Domain: Analytical Chemistry, Demonstrations, Physical Chemistry, Public Understanding /
Outreach. Topics: Acids / Bases, Fluorescence Spectroscopy, Natural Products, pH, Solutions /
Solvents, UV-Vis Spectroscopy. Additional Keywords (not in JCE keyword list): Narra,
Pterocarpus indicus, Lignum nephriticum, fluorescent acid-base indicator, isoflavone,
formononetin, Stokes law
1
This article describes a simple but visually striking demonstration of fluorescence from the
aqueous extract of the tropical hardwood Pterocarpus indicus, also known as narra. Its
simplicity and dramatic pH-dependence makes this demonstration particularly attractive as an
example of a natural substance that is strongly fluorescent. The phenomenon also has historical
significance because it represents the first recorded observation of fluorescence, noted over 400
years ago when the wood was known as Lignum nephriticum. In addition to the demonstrations
described in this article, a companion JCE Classroom Activity gives the details for a hands-on
activity based on narra extract fluorescence (1).
Fluorescence occurs when a substance absorbs light in one wavelength region and at the same
time emits light in a different region of the spectrum, normally of longer wavelength.
Fluorescent substances are encountered on a daily basis in modern society, where fluorescence is
used to capture our attention: traffic safety cones, school-crossing signs, store price labels, and
retail packaging. Here a fluorescent pigment absorbs ultraviolet and visible light from the sky or
room lights and emits visible light in a different
wavelength region from that absorbed. We notice
the object in part because it has a luminosity that is
enhanced relative to the non-fluorescent objects
around it. The characteristic shift to longer
wavelength from absorbed light to emitted light is
known as the Stokes shift.
The topic of fluorescence engages not just everyday uses, but also many important scientific
applications. Three significant examples are green
fluorescent protein, which is used as an important
biochemical tool for research involving gene
expression (2); automated DNA sequencing based
on fluorescence, which has significantly
accelerated the human genome project; and enzyme
linked immunoassay (ELISA) with fluorescence
detection, which plays a role in detection of mad
cow disease. Therefore, even brief introductions to
the topic of fluorescence can tie into exciting
developments in research.
While most of the fluorescent objects placed in our daily path contain synthetic fluorescent
pigments, there are good examples of natural substances that are fluorescent. A recent JCE
Chemistry Activity (3), which focused on fluorescence and other kinds of luminescence,
included the fluorescence of chlorophyll, quinine (found in tonic water) and specimens of
minerals. Another good example is riboflavin (vitamin B2) in aqueous solution, which gives
bright yellow fluorescence under a black light. While the observation of chlorophyll
fluorescence usually involves extraction by a flammable solvent, this demonstration involves
aqueous extraction of narra wood.
The place of fluorescence in a college chemistry curriculum is most likely in analytical and
physical chemistry. However, this demonstration can play a role in any part of the curriculum
that may touch on fluorescence. It can tie into demonstrations of the pH dependence of solutions
2
such as indicators or serve as an entry in discussions of the interaction of light, color and
molecules. Fluorescence in general plays such an important role in science and society that it
deserves more coverage in the high school and college chemistry curriculum.
Demonstration and Discussion
Pterocarpus indicus is a large, rainforest tree from Southeast Asia, the Philippines, and
Malaysia, which produces a choice hardwood used for furniture, cabinetry, and carving. It is
known by a variety of common names besides narra including angsana, Malay padauk, and
Amboyna burl. The wood can be purchased from a number of companies in the USA that supply
exotic wood species to wood workers. A small piece of wood about 24” by 2” x 2” will provide
enough shavings for many demonstrations and experiments, and pieces of about this size can be
purchased for under $15 including shipping costs1. Not all wood supply businesses are careful to
include the species name in their product description. For example, a wood simply called
padauk turned out to be Pterocarpus soyaxii (African padauk) from Cameroon, a closely related
species that produces a significantly weaker fluorescence in water. However, wood samples
with the common names listed above, particularly samples that were listed specifically as
Pterocarpus indicus, give good results. Eysenhardtia polystachya is another wood species tied
historically to narra with similar fluorescence properties. It is a shrub or small tree native to
Mexico and Southwest USA, also known as Mexican kidneywood. This wood apparently has
much less commercial value because a web search on this species yielded botanical information
but no commercial sources.
Observing the fluorescence from an aqueous infusion of narra is very straightforward and
produces eye-catching results. Adequate shavings of the yellowish-red, fragrant hardwood can
be obtained with a knife. However, it is easier to generate a large amount of shavings by using a
wood plane. A single shaving (~ 0.1 g) placed in 50 mL of room temperature tap water for a few
minutes is sufficient to produce a solution that exhibits blue fluorescence2. The aqueous solution
becomes a yellow to amber color. The shavings can be left in the solution, or removed by
decanting or filtering. The blue tinge from fluorescence is more easily seen in a concentrated
solution (3 grams of shavings placed in 200 mL of room temperature water with a drop of 1M
NaOH for at least an hour). The blue fluorescence is
easily observed by eye at the surface of the solution
from at least four different excitation sources:
sunlight, fluorescent room light, a long-wave
ultraviolet lamp (black light), and an ultraviolet light
emitting diode 3 (maximum wavelength, ~400 nm).
While fluorescence in general is more easily observed
in a darkened room, narra extract fluorescence is
bright enough to be viewed satisfactorily in normal
room light when excited using a UV LED. Pouring
the amber solution in direct sunlight while viewing
the flowing liquid against a dark background can
produce the spectacular effect of a blue stream.
Although the fluorescent substance in the extract is
susceptible to degradation in sunlight, a glass vessel
of the liquid sitting in a sunny location provides a
3
great show for several weeks and makes a wonderful discussion starter on the topic of
fluorescence. A solution kept out of direct sunlight at room temperature will remain fluorescent
for months.
The fluorescence is strongly pH dependent with the transition occurring from a pH of 6 (nonfluorescent) to 8 (fluorescent). Figure 1 shows a pH series of solutions in room light and under
black light illumination. The visible absorption spectrum of the narra solution changes with pH
as well, the solution becoming darker yellow in color as it becomes more fluorescent. For a
spectacular show, illuminate a sample of the non-fluorescent solution (pH below 6) with a black
light in a darkened room and add a small amount of base to the solution (a single drop of 0.1M
NaOH is usually sufficient); the solution becomes brightly fluorescent. Figure 1 also shows that
there is a second transition from fluorescent to
non-fluorescent at high pH, between 11 and 13.
Thus, it is possible with a strong base to raise
the pH sufficiently to achieve a non-fluorescent
result. The water-soluble constituents of the
wood will lower the pH of deionized water
enough that it normally requires raising the pH
to observe strong fluorescence. On the other
hand, tap water may have enough natural buffer
capacity to moderate the pH change and may
not require any pH adjustment for good
Figure 1 pH dependence of the aqueous extract of
fluorescence. Carrying out side-by-side
narra. Standard 1-cm cuvettes are labeled with the
extractions with deionized water and tap water
pH. The top series (A) shows the room light
can be an entry point to discussing the
appearance with a transition from nearly colorless to
difference between these two types of water; tap
yellow occurring between pH 6 and 8. The bottom
series (B) shows the same set of cuvettes under
water contains dissolved solids that buffer the
black light ultraviolet illumination with a transition
effect of adding narra shavings. The aqueous
from non-fluorescence to blue fluorescence
extract from African padauk exhibits a similar,
(maximum emission at 465 nm) occurring from pH
pH dependent, blue fluorescence, but it has a
6 to 8 and a second transition from fluorescent to
much weaker intensity than narra for the same
non-fluorescent from pH 11 to 13.
amount of wood.
Use of this demonstration in a small lecture forum (circa 50 students) can be done in more than
one way. Use of a concentrated solution in its fluorescent form (see above) on an overhead
projector will show the visible color in the projected image, and the blue fluorescence excited by
the projector lamp is visible at the bottom of the beaker. Lowering the pH will show the loss of
fluorescence and the change of color. In a more hands-on approach, the fluorescent and nonfluorescent narra solution can be placed in labeled cuvettes and passed among the students along
with a UV LED key-chain flashlight3 for excitation. The two samples are easily distinguished by
the bright blue fluorescence in the sample with elevated pH.
The Stokes shift, one of the key features of fluorescence, can be demonstrated quite easily using
a yellow filter. The Roscolene Medium Lemon yellow filter #8064 is an ideal filter because its
absorbance spectrum coincides very well with the absorbance spectrum of the fluorescent
component of narra. Placing the filter between the excitation source and the solution blocks the
fluorescence because the filter blocks the wavelengths needed for fluorescence. When the filter
4
is placed between the solution and the observer, the fluorescence is still visible because the filter
passes light emitted as fluorescence. Figure 2 shows the four spectra involved in this
demonstration: (a) the UV emission
spectrum of the black light, (b) the
fluorescence excitation spectrum, which
corresponds to the portion of the
absorption spectrum that leads directly to
fluorescence, (c) the fluorescence
emission spectrum, and (d) the
transmission spectrum of the yellow
filter. The yellow filter absorbs the black
light emission, but passes the narra
solution emission. Based on the spectra,
it appears that the filter absorbs a
significant amount of the fluorescent
emission, however to the human eye the
fluorescence appears to be undiminished.
Figure 2. Spectra related to narra extract fluorescence. (A)
A careful observer will note a subtle shift
The black light emission spectrum, dot-dash line (48” black
in the color emitted from blue to green,
light fluorescent tube, Phillips F40T12/BLB); (B) the narra
because the filter removes a portion of the solution fluorescence excitation spectrum, solid line (the
excitation spectrum indicates where the solution absorbs light
blue light from the emission. It is clear
that leads to fluorescence); (C) the narra solution fluorescence
from these simple observations that the
emission spectrum, dotted line; and (D) the percent
light emission is in a different wavelength transmission spectrum of the yellow filter (Roscolene
region from the light absorbed, and
Medium Lemon #806), dashed line. The excitation spectrum
because blue light has longer wavelengths and both emission spectra are normalized and refer to the left
axis; the filter spectrum refers to the right axis. The yellow
than violet light it illustrates the Stokes
filter absorbs the black light emission, but passes the narra
shift.
solution emission.
Historical Background
According to Partington (4), the first recorded observation of fluorescence dates to the year
15655. The writings of the Spanish physician and botanist Nicolás Monardes describe a wood
from New Spain called Lignum nephriticum, which was used as a medical treatment for liver and
kidney ailments. Monardes also mentions that water in contact with the wood assumes an
unusual blue color. Lignum nephriticum was well known in sixteenth and seventeenth century
Europe due to cups or challises made of this exotic wood, which likewise exhibited a curious
blue color at the surface of water placed in the cup. Robert Boyle in 1664 reported that the blue
tinge was dependent on pH (4). Sir Isaac Newton in 1672 included his observations of the
phenomenon in his efforts to formulate his theory of light and color (5). However, the botanical
origin of Lignum nephriticum was lost by the mid eighteenth century.
Safford’s work in 1915 reestablished the origins of Lignum nephriticum (7). There are two
different woods that became confused as one, both giving similar fluorescent results in water.
The two woods are narra and Mexican kidneywood mentioned earlier in this article. Safford is
convinced that Robert Boyle’s observations were based on Mexican kidneywood, whereas the
once-famous cups came from the larger narra logs, which were imported to Spain from the
5
Phillipines via Mexico, no doubt contributing to the confusion of origins. The two species are in
the same botanical family, Fabaceae (formerly known as Leguminosae).
Published studies of the constituents of the woods, narra and Mexican kidneywood, have
suggested the principal fluorescent component is an isoflavone. Cooke and Rae examined the
heartwood of narra (8) and, while their study made no mention of the fluorescence
characteristics, they did identify 7-hydroxy-4’methoxyisoflavone (formononetin, I) as one of
several principal components in successive
petroleum and acetone extractions. Burns et al.
sought Robert Boyle’s fluorescent indicator in
their study of Mexican kidneywood (9). They
assigned this role to 7-hydroxy-2’, 4’, 5’trimethoxyisoflavone, which they identified in a
(I) 7-hydroxy-4’-methoxyisoflavone or
formononetin
methanol extract. In the Mexican kidneywood
study, they specifically examined the constituents
for their fluorescent characteristics, including pH dependence. Because the two isoflavones are
so closely related, Burns et al. assume that formononetin is the fluorescent constituent in narra,
and that the similarity explains the like accounts of blue fluorescence from the two woods.
Preliminary studies of narra and formononetin fluorescence done in our laboratories, however,
suggest that the identity of the fluorescent component is not completely known. We have
confirmed that formononetin, a substance that is only partially soluble in room-temperature
water, does indeed give a blue fluorescent aqueous solution with a similar pH-dependence to
narra. Our results are in agreement with published studies on formonentin fluorescence (10, 11).
On the other hand, an aqueous formonentin solution absorbs light only in the UV region and
hence is colorless. In our study, the fluorescence excitation spectrum of narra extract, shown in
Figure 2B, clearly indicates that the fluorescent component absorbs in the region 370 to 450 nm
with peak absorbance at 430 nm accounting for some if not all of the extract’s yellow
appearance. The UV-Vis absorption spectrum of narra extract also shows the absorption band at
430 nm along with another band in the ultraviolet that does not lead to fluorescence; both
absorption bands are pH dependent. While formononetin may be related to the fluorescent
substance in narra wood, formononetin cannot by itself account for the fluorescence in narra
solutions.
Fluorescence within the family Fabaceae is not limited to the three species of wood already
mentioned. In his book on identifying woods, Hoadley (12) lists nine North American species in
the family Fabaceae for which it is known that the wood itself is fluorescent. Other members of
the Fabaceae family are known for containing isoflavones, notably red clover and soybeans. We
have confirmed in our laboratories that red clover extract, which contains four isoflavones
including formononetin, also exhibits a pH-dependent fluorescence similar to the narra extract.
The focus of this paper however is on the aqueous extract of narra wood since that was the
historical observation.
6
Summary
This paper describes an easy demonstration of the first recorded observation of fluorescence,
noted over 400 years ago. The narra wood is inexpensive and easily obtained. Its aqueous
extract gives a beautiful, strong, blue fluorescence in sunlight and under ultraviolet light. Narra
extract is one of the more easily demonstrated examples of fluorescence in natural substances
since the extract is so simple to prepare. The fact that the fluorescence is strongly pH dependent
provides an additional dimension of interest to the demonstration. Highlighting the historical
significance of Lignum nephriticum and discussing some of the relevant studies suggesting the
chemical origin of the fluorescence provides a context for this demonstration.
Acknowledgements
MM thanks Holly Hoffman, Ashlee Hardy,
Sarah Jelsema, and Rachael Glassford for
their work on this project, and the students
of two particular classes: the Calvin College
Interim 2004 course on fluorescence, and
the Fall 2005 Honors General Chemistry
Laboratory course.
Supplemental Material
A photo of narra wood and information on
wood sources, ultraviolet LEDs, and
additional spectral data on narra solutions is
available in this issue of JCE Online.
Footnotes
1.
One source is Curious Woods division of Curtis Lumber Company, Ballston Spa, NY. 1800-724-WOOD, http://www.curiouswoods.com/ (last accessed 21-October-05).
Information on additional vendors of narra used by the author is available in the online
supplement.
2.
It is important to keep in mind that the fluorescence is pH dependent. While using tap
water of sufficient hardness will normally exhibit fluorescence without pH adjustment, it
is possible that raising the pH will be necessary to see the full effect.
3.
The author has used UV light emitting diodes with maximum emission at 395, 400, and
405 nm; all work well. See online supplement for more information. As with any light
source containing UV wavelengths, appropriate caution should be used to minimize
direct exposure. Never look directly at a bright light source.
4.
See filter manufacturer for directory of dealers, Rosco USA, (800) 767-2669,
http://www.rosco.com/us/ (last accessed 21-October-05). You can also web search on
“roscolene filter sheets” to find vendors. See online supplement for additional info. Note
that other yellow filters are effective, particularly with the UV LED, but the Roscolene
#806 filter is clearly the best at completely blocking fluorescence from a black light.
5.
Authors give slightly different dates for Monardes’ writings: 1565 according to
Partington (4), Valuer (6) and Safford (7); 1574 according to Harvey (5), which
7
according to Partington corresponds to the first appearance of the Latin translation from
the earlier Spanish version.
6.
The online supplement has more information about the excitation spectrum of narra
extract.
Literature Cited
1.
Muyskens, M. A. J. Chem. Educ. (companion article-narra Chem Activity)
2.
Hicks, B. W. J. Chem. Educ. 1999, 76, 409-415.
3.
O'Hara, P. B.; Engelson, C.; St. Peter, W. J. Chem. Educ. 2005 82 48A-48B & 49-52.
4.
Partington, J. R. Ann. of Sci. 1955, 11, 1-26.
5.
Harvey, E. N. A History of Luminescence From the Earliest Times Until 1900; The
American Philosophical Society: Philadelphia, 1957; pp 391-392.
6.
Valeur, B.; Molecular Fluorescence: Principles and Applications; Wiley-VCH:
Weinheim, Germany, 2002; p 6.
7.
Safford, W. E., Annual Report of the Board of Regents of the Smithsonian Institution
1915, 271-298.
8.
Cooke, R. G.; Rae, I. D. Aust. J. Chem. 1964, 17, 379-384.
9.
Burns, D. T.; Dalgarno, B. G.; Gargan, P. E.; Grimshaw, J., Phytochemistry 1984, 23,
167-169.
10.
Dunford, C. L; Smith, G. J.; Swinny, E. E.; Markham, K. R. Photochem. Photobiol. Sci.
2003, 2, 611-615.
11.
de Rijke, E.; Joshi, H. C.; Sanderse, H. R.; Ariese, F.; Brinkman, U. A. T.; Gooijer, C.
Analytica Chemica Acta 2002, 468, 3-11.
12.
Hoadley, B. R.; Identifying Wood: Accurate Results with Simple Tools; Taunton Press:
Newtown, CT, 1990; p 53.
Notes to additional photos:
Page 1: Blue fluorescence of an aqueous extract of narra excited by a 400-nm UV LED in room
light. The beaker is viewed from above and sits partially over a white surface to show the yellow
appearance of the solution. The beaker contains one wood shaving of narra; Page 2: The amber,
aqueous extract of narra wood appears blue in direct sunlight; Page 3: Another illustration of
illumination of narra extract by direct sunlight; Page 7: Under black light (long-wave UV light),
pH>7 on left, pH<5 on right.
8
Supplemental material for online publication to “The fluorescence of Lignum nephriticum: a
flash back to the past, and a simple demonstration of natural substance fluorescence.”
In this supplement:
(1) Additional photos (photos not used in print can be included here)
(2) Commercial sources used for narra (Pterocarpus indicus) samples
(3) Description of the UV LED light sources
(4) Additional spectral information on narra solutions including commercial source for the yellow filter material
(1) Additional photos
Narra wood. The bottom piece is 24” x 3” x 3/4” from woodcraft.com. The curved edge is due to making shavings
using a wood plane.
(2) Commercial sources used for narra (Pterocarpus indicus) samples
Wood notes: I have observed that fresh shavings of wood are more potent than shavings stored for a long period of
time (1.5 years). I prefer straight-grained wood to burl wood because the former is easier to make into shavings
with a wood plane. Burl wood has a gnarled grain coming from a knot in the wood. I have observed however that
the burl wood gives a somewhat more concentrated extract for the same mass of shavings.
Be aware that some vendors may have a minimum order – none of the ones listed below had a minimum order at the
time I placed the order. Also, note that there is some volatility in the web-based market for exotic woods; within
one year, two of my sources for straight-grained narra became unavailable. I have replaced them with two more
sources listed below.
Curious Woods division of Curtis Lumber Company
http://www.curiouswoods.com/ (last accessed 21-October-05)
1-800-724-WOOD
Ballston Spa, NY.
Product description: Narra (link was on first page of web site, no dimensions were listed for the piece of wood but
the shipped piece was generous in size, and the price was inexpensive)
9
Woodcraft Supply Corp.
http://www.woodcraft.com (last accessed 21-October-05)
800-225-1153
Parkersburg, WV 26104
Product description: Narra (NOTE: the company no longer carries this item, noted as of 24-July-05, P. indicus is
however available from this company as amboyna burl – see next product)
Product description: Amboyna burl pen blank - Note: a burl pen blank is a small, inexpensive piece of wood.
Exotic Wood Group
http://www.exoticwoodgroup.com (last accessed 21-October-05)
(631) 262-8825
Northport, NY 11768
Product description: Amboyna Burl pen blank
Blankity-Blanks (Div. of Bronwyn, Inc.)
12285 Trautwein Road
Austin, Texas 78737
1-(866) 744- WOOD (9663) Toll Free Order Line
http://www.blankity-blanks.com/ (last accessed 21-October-05)
Product description: Pen blanks available in three different grains of narra, for example, “Narra, Bee's-Wing
Figure”, also pen blank available in Amboyna Burl – all listed as Pterocarpus indicus. Note: author has located this
source but not personally ordered from it as of the date of publication.
(3) Description of the UV LED light source
The UV LED is fantastic for exciting many common fluorescent substances such as yellow highlighter ink. It
doesn’t work for quinine, but works very well for narra, chlorophyll, and green, yellow, orange and red/magenta
fluorescent inks and pigments. The narra extract blue fluorescence is easily visible in room light with UV LED
illumination, whereas the black light requires a semi-darkened room for best effect. UV LEDs are available in
wavelengths of 395, 400, and 405 nm depending on the vendor (and recently 375 nm), but as wavelength gets
shorter brightness diminishes.
There are two options for obtaining a UV LED as an excitation source.
1) For under $16 each, buy a keychain UV LED flashlight from a commercial vendor. This is a really fun tool for
the fluorescence enthusiast. I carry it with me and check out items that catch my eye in a store to see if they are
indeed fluorescent. It turns out that there are many vendors since these are popular party lights.
I have used the Photon Micro-light II – Note: the UV or “purple” color (405 nm peak wavelength) is not available
from every vendor of this line of keychain LED lights, but it is not very hard to find the violet kind. It uses 2
CR2016 batteries for 12-14 hours of continuous use. It is bright, quite robust, and has a locked-on setting.
- Manufacturer’s website: List price is $15.95.
URL=(http://www.photonlight.com/products/photon_microlight_II.html, last accessed 21-October-05)
- Educator’s Innovations (http://www.teacherssource.com) carries the Micro-light II at the list price.
- The author purchased his Micro-light II for less than the list price from URL=(http://www.basegear.com, last
accessed 21-October-05) in January 2005, and a second from URL=(http://www.alphanetproducts.com, last
accessed 21-October-05) in May 2005.
- There are also other UV LED keychain flashlights available for under $3 each. I ordered some from a source on
www.ebay.com in August 2005. These were football shaped with unusual batteries, and came with a full set of
spare batteries. They have only a momentary-on push button, and are not as bright as the Micro-light II.
2) The other option is to buy the components to power a UV LED. This was attractive price-wise even in 2004, but
the dropping price of keychain UV LEDs makes this advantage less clear. The other advantage of this approach is
that you can power any color LED.
The author ordered 400-nm and 395-nm light emitting diodes from http://www.LEDSupply.com (last accessed 21October-05)
10
Both 5mm LEDs, 15 degree angle of illumination
395 nm wavelength maximum, Part number: L4-1-U5TH15-1
400 nm wavelength maximum, Part number: L3-0-U5TH15-1
under $2 each as of 21-October-05, discounts for volumes over 10.
Both wavelengths perform similarly.
There are two options for powering a UV LED:
1) We power the LED using a 9-volt battery and a simple, homebuilt cable consisting of three components: a 9-V
battery connector, a cable with an LED socket (Newark part number 16F6478), and a 470-Ohm (1/4 watt) current
limiting resistor. The resistor is placed between either one of the two connections between battery connector and
socket. Total length of cable assembly is 12 inches.
We give the cable a durable finish by encasing the wires on either side of the resistor and then the resistor and solder
points in heat shrink tubing – three overlapping pieces. Total cost of the cable parts is under $4 a piece. The LED is
a diode, so if you plug it into the cable backwards, no current flows and no harm is done – reversing the LED puts it
in the correct orientation to give light.
One large advantage of using the cable is that you can use it with other LEDs. You can have a collection of LEDs
and do experiments with blue (which shows some fluorescence very nicely and is brighter than the UV LED), green,
yellow, red, and white. The white LED currently functions by using a blue LED along with a phosphor that absorbs
some of the blue emission and fluoresces yellow, where the mixture of yellow and blue light gives white. This can
be observed by recording the emission spectrum of the white LED, which reveals two bands – blue and yellow.
Another application of fluorescence!
2) A second method for powering an LED is described in a J. Chem. Educ. article on LEDs - see Lisensky, George
C.; Condren, S. Michael; Widstrand, Cynthia G.; Breitzer, Jonathan; Ellis, Arthur B. “LEDs Are Diodes”, J. Chem.
Educ. 2001, 78, 1664A.
(3) Additional spectral information on narra solutions
(a) Fluorescence excitation and emission spectrum.
Fluorescence
At Calvin College we have recorded the excitation and emission spectrum of narra extract at selected pH values
using a fluorimeter (Aminco Bowman Series 2 Luminescence Spectrometer). The excitation spectrum involves
setting a light detector to monitor emitted light at 465 nm (the wavelength of maximum emission) while scanning
the excitation wavelength over a spectral range from 250 to 450 nm. When the excitation wavelength is in a region
where the fluorescent substance in narra absorbs light
Effect of pH on Emission and Excitation
then we observe light being emitted at 465 nm.
Thus, the excitation spectrum reveals the absorption
12
spectrum of the fluorescent substance. The emission
Excitation
Emission
10
spectrum involves setting the excitation light
8
wavelength at one that is absorbed strongly by the
6
fluorescent substance, in this case 400 nm, and
4
scanning the wavelength observed by the light
detector over a spectral region from 415 nm to 650
2
nm. Here, we observe the spectral distribution of the
0
200
300
400
500
600
emitted fluorescence. We observe that the
Wavelength (nm)
fluorescent substance in narra aqueous solutions
absorbs light from 390 to 450 nm (full-width at halfpH 5.56
pH 6.54
pH 7.62
maximum) with the maximum absorbance at 430 nm.
The emission spectrum is from 450 to 495 nm
Figure 1 - excitation and emission spectra of narra aqueous
(FWHM) with maximum emission at 465 nm.
extract at selected pH values.
11
Figure 1 shows the excitation and emission spectra of narra solutions at selected pH values. The excitation
spectrum shows that the solution absorbs increasing amounts of light near 430 nm as pH increases, corresponding to
increasing amounts of emission near 465 nm as pH increases. Note: the excitation spectra have higher intensity than
the emission spectra because we chose to set the detector sensitivity at 400-nm excitation (465-nm emission) rather
than use 430-nm excitation, which corresponds to the absorption maximum. Had we set the detector sensitivity
using 430-nm excitation the excitation and emission peaks would have been the same height. Exciting at 400 nm
allows us a wider wavelength range for scanning over the entire emission spectrum. Detector sensitivity was left
unchanged for all of the spectra shown in figure 1.
(b) UV-Vis absorption spectrum.
Abs
Figure 2 shows absorbance spectra collected on a UV-Vis absorption spectrophotometer (Varian Cary 100 Bio).
This figure also shows the absorption band at 430 nm growing in as pH increases. An accompanying absorption
band at 338 nm grows in over the same pH range
but apparently does not lead to fluorescence
Effects of pH on Absorbance
because there is no similar peak in the excitation
spectrum. The absorbance between 400 and 450
1
nm accounts for the yellow appearance of the
0.8
fluorescent narra solution. Note that while the
concentration of the extract is unknown, for these
0.6
fluorescence and absorbance measurements we
0.4
used dilute narra solution with absorbance values
below about 0.5 absorbance units (for
0.2
wavelengths above 320 nm).
(Note: special thanks go to Calvin College
undergraduate Ashlee Hardy for recording the
spectra in Figures 1 and 2 and preparing the
figures.)
(c) The yellow filter.
0
300
400
500
Wavelength (nm)
pH 4.52
pH 7.05
600
pH 10.38
Figure 2 – UV-Vis absorbance spectra of narra solutions at
selected pH values.
Filter source: The author ordered Roscolene Medium Lemon #806, 20 x 24 inch sheet from Theatre House, Inc.,
Covington, KY, (800) 827-2414 URL=(http://www.theatrehouse.com, last accessed 21-October-05). The web site
had a $10 minimum order and including shipping and the under minimum order charge ($3), the cost was around
$15.
The manufacturer’s web site includes links to find local dealers (http://www.rosco.com/us/, last accessed 21October-05).
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