<PAGE>

                                                                   EXHIBIT 4.28

                            8% CONVERTIBLE DEBENTURE

         NEITHER THESE SECURITIES NOR THE SECURITIES ISSUABLE UPON CONVERSION
         HEREOF HAVE BEEN REGISTERED WITH THE UNITED STATES SECURITIES AND
         EXCHANGE COMMISSION OR THE SECURITIES COMMISSION OF ANY STATE OR UNDER
         THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"). THE SECURITIES ARE
         RESTRICTED AND MAY NOT BE OFFERED, RESOLD, PLEDGED OR TRANSFERRED
         EXCEPT AS PERMITTED UNDER THE ACT PURSUANT TO AN EFFECTIVE REGISTRATION
         STATEMENT OR AN EXEMPTION FROM SUCH REGISTRATION REQUIREMENTS.

NO.                                                       US $

                          NHANCEMENT TECHNOLOGIES INC.

                    8% CONVERTIBLE DEBENTURE DUE MAY 30, 2001

                  THIS DEBENTURE is issued by NHancement Technologies Inc., a
corporation organized and existing under the laws of the State of Delaware (the
"Company") and is designated as its 8% Convertible Debenture Due May 30, 2001.

                  FOR VALUE RECEIVED, the Company promises to pay to [ ] or
permitted assigns (the "Holder"), the principal sum _______________ and 00/100
(US $xx,000) Dollars on May 30, 2001 (the "Maturity Date") and to pay interest
on the principal sum outstanding from time to time quarterly in arrears at the
rate of 8% per annum accruing from the date of initial issuance. Accrual of
interest shall commence on the first business day to occur after the date of
initial issuance and continue until payment in full of the principal sum has
been made or duly provided for. Quarterly interest payments shall be due and
payable on July 1, October 1, January 1 and May 1 of each year, commencing with
July 1, 2000. If any interest payment date or the Maturity Date is not a
business day in the State of New York, then such payment shall be made on the
next succeeding business day. The Company will pay the principal of and any
accrued but unpaid interest due upon this Debenture on the Maturity Date, less
any amounts required by law to be deducted, to the registered holder of this
Debenture and addressed to such holder at the last address appearing on the
Debenture Register. The forwarding of such check, shall constitute a payment of
principal and interest hereunder and shall satisfy and discharge the liability
for principal and interest on this Debenture to the extent of the sum
represented by such check plus any amounts so deducted.

                  This Debenture is subject to the following additional
provisions:

                           1.       The Company shall be entitled to withhold
from all payments of interest on this Debenture any amounts required to be
withheld under the applicable provisions of the United States income tax laws or
other applicable laws at the time of such payments, and Holder shall execute and
deliver all required documentation in connection therewith.

                           2.       This Debenture has been issued subject to
investment representations of the original purchaser hereof and may be
transferred or exchanged only in compliance with the Securities Act of


                                       1
<PAGE>

1933, as amended (the "Act"), and other applicable state and foreign securities
laws. The Holder shall deliver written notice to the Company of any proposed
transfer of this Debenture. In the event of any proposed transfer of this
Debenture, the Company may require, prior to issuance of a new Debenture in the
name of such other person, that it receive reasonable transfer documentation
including legal opinions that the issuance of the Debenture in such other name
does not and will not cause a violation of the Act or any applicable state or
foreign securities laws. Prior to due presentment for transfer of this
Debenture, the Company and any agent of the Company may treat the person in
whose name this Debenture is duly registered on the Company's Debenture Register
as the owner hereof for the purpose of receiving payment as herein provided and
for all other purposes, whether or not this Debenture be overdue, and neither
the Company nor any such agent shall be affected by notice to the contrary. This
Debenture has been executed and delivered pursuant to the Convertible Debenture
Purchase Agreement dated as of May __, 2000 between the Company and the original
Holder (the "Purchase Agreement"), and is subject to the terms and conditions of
the Purchase Agreement, which are, by this reference, incorporated herein and
made a part hereof. Capitalized terms used and not otherwise defined herein
shall have the meanings set forth for such terms in the Purchase Agreement.

                           3.       The Holder of this Debenture is entitled, at
its option, to convert at any time commencing on the date hereof, the principal
amount of this Debenture or any portion thereof, into shares of Common Stock of
the Company ("Conversion Shares") at a conversion price for each share of Common
Stock ("Conversion Price") equal to the lesser of $13.00 (subject to adjustment
for stock splits and the like) or (ii) 91% of the average of the three (3)
lowest closing bid prices during the ten (10) Trading Days immediately preceding
the Conversion Date; PROVIDED, HOWEVER, the Conversion Price shall not be less
than $8.00 (adjusted for stock splits and the like).

                           4.       The entire unpaid balance of this Debenture
and accrued interest thereon outstanding on the Maturity Date hereof shall be
due and payable without notice on the Maturity Date.

                           5.       The Company shall use at least 50% of the
net proceeds of any subsequent equity financing to redeem this Debenture as set
forth in this Section 5. The Company shall also have the right to deliver to the
Holder a written notice of the Company's intent to redeem all or any part of the
entire outstanding amount of this Debenture at a price of 105% of the
outstanding principal balance, plus all accrued but unpaid interest. The Company
shall make the redemption payment to the Holder six (6) Trading Days after the
notice of redemption is tendered ("Redemption Payment Date"), or else the
redemption notice shall be void, and the Company shall thereafter not have any
further right to redeem this Debenture. The Holder shall have the right to
convert this Debenture as set forth in Section 8 until the Trading Day prior to
the Redemption Payment Date.

                           6.       Notwithstanding anything to the contrary
contained herein, in the event that a conversion (when aggregated with all prior
conversions of portions of this Debenture) requires the Company to issue a
number of shares of Common Stock which would exceed 19.9% of the number of
shares of Common Stock issued and outstanding on the date of this Debenture, the
Company shall issue only such number of shares of Common Stock as shall not
exceed such limit and shall pay the Holder cash in the amount of the Volume
Weighted Price of the Common Stock as reported by Bloomberg L.P. on the
Conversion Date for the number of shares of Common Stock in excess of such
number of shares into which this Debenture (or the portion thereof then being
converted) is then convertible at the Conversion Price. Any payments under this
Paragraph 6 shall be made to an account designated in writing by the Holder to
the Company when the Notice of Conversion is given. The rights of all holders of
Convertible Debentures issued under the Purchase Agreement to convert their
Convertible Debentures into shares of Common Stock shall be prorated among such
holders based on their respective percentage holdings at the time of conversion


                                       2
<PAGE>

of the aggregate outstanding amount of all Convertible Debentures in order to
comply with the aforesaid overall limitation. Any conversion which is paid in
cash under this Paragraph 6 shall be paid within five (5) Trading Days of the
Conversion Date, or else the Company shall thereafter be unable to exercise its
redemption rights under Paragraph 5 with respect to the outstanding Debentures.

                           7.       Intentionally omitted.

                           8.       (a) Conversion shall be effectuated by
surrendering this Debenture to the Company (if such Conversion will convert all
outstanding principal) together with the form of conversion notice attached
hereto as Exhibit A (the "Notice of Conversion"), executed by the Holder of this
Debenture evidencing such Holder's intention to convert this Debenture or a
specified portion (as above provided) hereof, and accompanied, if required by
the Company, by proper assignment hereof in blank. Interest accrued or accruing
from the date of issuance to the date of conversion shall be paid in cash as set
forth above. No fraction of a share or scrip representing a fraction of a share
will be issued on conversion, but the number of shares issuable shall be rounded
to the nearest whole share. The date on which Notice of Conversion is given (the
"Conversion Date") shall be deemed to be the date on which the Holder faxes the
Notice of Conversion duly executed to the Company. Facsimile delivery of the
Notice of Conversion shall be accepted by the Company at facsimile number (925)
251 3203 Attn.: Douglas S. Zorn. Certificates representing Common Stock upon
conversion will be delivered to the Holder within three (3) Trading Days from
the date the Notice of Conversion is delivered to the Company. Delivery of
shares upon conversion shall be made to the address specified by the Holder in
the Notice of Conversion.

                                    (b) The Company understands that a delay in
the issuance of shares of Common Stock upon a conversion beyond the three (3)
Trading Day period described in Paragraph 8(a) could result in economic loss to
the Holder. As compensation to the Holder for such loss, the Company agrees to
pay late payments to the Holder for late issuance of shares of Common Stock upon
conversion in accordance with the following schedule (where "No. Trading Days
Late" is defined as the number of Trading Days beyond three (3) Trading Days
from the date the Notice of Conversion is delivered to the Company).

<TABLE>
<CAPTION>
------------------------------------------------------------------------------------------------
           No. Trading Days Late                               Late Payment for Each
                                                             $5,000 of Principal Amount
                                                                  Being Converted
------------------------------------------------------------------------------------------------
<S>                                                          <C>
                     1                                                  $100
------------------------------------------------------------------------------------------------
                     2                                                  $200
------------------------------------------------------------------------------------------------
                     3                                                  $300
------------------------------------------------------------------------------------------------
                     4                                                  $400
------------------------------------------------------------------------------------------------
                     5                                                  $500
------------------------------------------------------------------------------------------------
                     6                                                  $600
------------------------------------------------------------------------------------------------
                     7                                                  $700
------------------------------------------------------------------------------------------------
                     8                                                  $800
------------------------------------------------------------------------------------------------
                     9                                                  $900
------------------------------------------------------------------------------------------------
                    10                                                 $1,000
------------------------------------------------------------------------------------------------
               More than 10                              $1,000 +$200 for each Trading Day
                                                            Late beyond 10 Trading Days
------------------------------------------------------------------------------------------------

------------------------------------------------------------------------------------------------
</TABLE>

                           The Company shall pay any payments incurred under
this Paragraph 8(b) in immediately available funds upon demand. Nothing herein
shall limit Holder's right to pursue injunctive


                                       3
<PAGE>

relief and/or actual damages for the Company's failure to issue and deliver
Common Stock to the holder, including, without limitation, the Holder's actual
losses occasioned by any "buy-in" of Common Stock necessitated by such late
delivery. Furthermore, in addition to any other remedies which may be available
to the Holder, in the event that the Company fails for any reason to effect
delivery of such shares of Common Stock within three (3) Trading Days from the
date the Notice of Conversion is delivered to the Company, the Holder will be
entitled to revoke the relevant Notice of Conversion by delivering a notice to
such effect to the Company, whereupon the Company and the Holder shall each be
restored to their respective positions immediately prior to delivery of such
Notice of Conversion, and in such event no late payments shall be due in
connection with such withdrawn conversion.

                           (c)      If at any time (a) the Company challenges,
disputes or denies the right of the Holder to effect the conversion of this
Debenture into Common Stock or otherwise dishonors or rejects any Notice of
Conversion properly delivered in accordance with this Paragraph 8 or (b) any
Company stockholder who is not and has never been an Affiliate (as defined in
Rule 405 under the Securities Act of 1933, as amended) of the Holder obtains a
judgment or any injunctive relief from any court or public or governmental
authority which denies, enjoins, limits, modifies, delays or disputes the right
of the holder hereof to effect the conversion of this Debenture into Common
Stock, then the Holder shall have the right, by written notice, to require the
Company to promptly redeem this Debenture for cash at a redemption price equal
to one hundred ten percent (110%) of the outstanding principal amount hereof and
all accrued and unpaid interest hereon. Under any of the circumstances set forth
above, the Company shall be responsible for the payment of all costs and
expenses of the Holder, including reasonable legal fees and expenses, as and
when incurred in disputing any such action or pursuing its rights hereunder (in
addition to any other rights of the Holder), subject in the case of clause (b)
to the Company's right to control and assume the defense of any such action. In
the absence of an injunction precluding the same, the Company shall issue shares
upon a properly noticed conversion.

                           9.       No provision of this Debenture shall alter
or impair the obligation of the Company, which is absolute and unconditional, to
pay the principal of, and interest on, this Debenture at the time, place, and
rate, and in the coin or currency herein prescribed. This Debenture is a direct
obligation of the Company.

                           10.      No recourse shall be had for the payment of
the principal of, or the interest on, this Debenture, or for any claim based
hereon, or otherwise in respect hereof, against any incorporator, shareholder,
employee, officer or director, as such, past, present or future, of the Company
or any successor corporation, whether by virtue of any constitution, statute or
rule of law, or by the enforcement of any assessment or penalty or otherwise,
all such liability being, by the acceptance hereof and as part of the
consideration for the issue hereof, expressly waived and released.

                           11.      If the Company merges or consolidates with
another corporation or sells or transfers all or substantially all of its assets
to another person and the holders of the Common Stock are entitled to receive
stock, securities or property in respect of or in exchange for Common Stock,
then as a condition of such merger, consolidation, sale or transfer, the Company
and any such successor, purchaser or transferee agree that the Debenture may
thereafter be converted on the terms and subject to the conditions set forth
above into the kind and amount of stock, securities or property receivable upon
such merger, consolidation, sale or transfer by a holder of the number of shares
of Common Stock into which this Debenture might have been converted immediately
before such merger, consolidation, sale or transfer, subject to adjustments
which shall be as nearly equivalent as may be practicable. In the event of any
proposed merger, consolidation or sale or transfer of all or substantially all
of the assets of the Company (a "Sale"), the Holder hereof shall have the right
to convert by delivering a Notice of Conversion to the


                                       4
<PAGE>

Company within fifteen (15) days of receipt of notice of such Sale from the
Company. In the event the Holder hereof shall elect not to convert, the Company
may prepay all outstanding principal and accrued interest on this Debenture as
provided in Section 5, less all amounts required by law to be deducted, upon
which tender of payment following such notice, the right of conversion shall
terminate.

                           12.      The Holder of the Debenture, by acceptance
hereof, agrees that this Debenture is being acquired for investment and that
such Holder will not offer, sell or otherwise dispose of this Debenture or the
Shares of Common Stock issuable upon conversion thereof except under
circumstances which will not result in a violation of the Act or any applicable
state Blue Sky or foreign laws or similar laws relating to the sale of
securities.

                           13.      This Debenture shall be governed by and
construed in accordance with the laws of the State of New York. Each of the
parties consents to the jurisdiction of the federal courts whose districts
encompass any part of the City of New York or the state courts of the State of
New York sitting in the City of New York in connection with any dispute arising
under this Agreement and hereby waives, to the maximum extent permitted by law,
any objection, including any objection based on forum non conveniens, to the
bringing of any such proceeding in such jurisdictions.

                           14.      The following shall constitute an "Event of
                                    Default":

                           a.       The Company shall default in the payment of
                                    principal or interest on this Debenture and
                                    same shall continue for a period of three
                                    (3) days; or

                           b.       Any of the material representations or
                                    warranties made by the Company herein, in
                                    the Purchase Agreement, the Registration
                                    Rights Agreement, or in any agreement,
                                    certificate or financial statements
                                    heretofore or hereafter furnished by the
                                    Company in connection with the execution and
                                    delivery of this Debenture or the Purchase
                                    Agreement shall be false or misleading in
                                    any material respect at the time made; or

                           c.       The Company fails to issue shares of Common
                                    Stock to the Holder or to cause its Transfer
                                    Agent to issue shares of Common Stock upon
                                    proper exercise by the Holder of the
                                    conversion rights of the Holder in
                                    accordance with the terms of this Debenture,
                                    fails to transfer or to cause its Transfer
                                    Agent to transfer any certificate for shares
                                    of Common Stock issued to the Holder upon
                                    conversion of this Debenture as and when
                                    required by this Debenture or the
                                    Registration Rights Agreement, and such
                                    transfer is otherwise lawful, or fails to
                                    remove any restrictive legend or to cause
                                    its Transfer Agent to transfer any
                                    certificate or any shares of Common Stock
                                    issued to the Holder upon conversion of this
                                    Debenture as and when required by this
                                    Debenture, the Purchase Agreement or the
                                    Registration Rights Agreement and such
                                    legend removal is otherwise lawful, and any
                                    such failure shall continue uncured for five
                                    (5) business days; or

                           d.       The Company shall fail to perform or
                                    observe, in any material respect, any other
                                    covenant, term, provision, condition,
                                    agreement or obligation of the Company under
                                    the Purchase Agreement, the Registration
                                    Rights Agreement or this Debenture and such
                                    failure shall continue uncured for a period
                                    of thirty (30) days after written notice
                                    from the Holder of such failure; or


                                       5
<PAGE>

                           e.       The Company shall (1) admit in writing its
                                    inability to pay its debts generally as they
                                    mature; (2) make an assignment for the
                                    benefit of creditors or commence proceedings
                                    for its dissolution; or (3) apply for or
                                    consent to the appointment of a trustee,
                                    liquidator or receiver for its or for a
                                    substantial part of its property or
                                    business; or

                           f.       A trustee, liquidator or receiver shall be
                                    appointed for the Company or for a
                                    substantial part of its property or business
                                    without its consent and shall not be
                                    discharged within sixty (60) days after such
                                    appointment; or

                           g.       Any governmental agency or any court of
                                    competent jurisdiction at the instance of
                                    any governmental agency shall assume custody
                                    or control of the whole or any substantial
                                    portion of the properties or assets of the
                                    Company and shall not be dismissed within
                                    sixty (60) days thereafter; or

                           h.       Any money judgment, writ or warrant of
                                    attachment, or similar process in excess of
                                    One Hundred Thousand ($100,000) Dollars in
                                    the aggregate shall be entered or filed
                                    against the Company or any of its properties
                                    or other assets and shall remain unpaid,
                                    unvacated, unbonded or unstayed for a period
                                    of sixty (60) days or in any event later
                                    than five (5) days prior to the date of any
                                    proposed sale thereunder; or

                           i.       Bankruptcy, reorganization, insolvency or
                                    liquidation proceedings or other proceedings
                                    for relief under any bankruptcy law or any
                                    law for the relief of debtors shall be
                                    instituted by or against the Company and, if
                                    instituted against the Company, shall not be
                                    dismissed within sixty (60) days after such
                                    institution or the Company shall by any
                                    action or answer approve of, consent to, or
                                    acquiesce in any such proceedings or admit
                                    the material allegations of, or default in
                                    answering a petition filed in any such
                                    proceeding; or

                           j.       The Company shall have its Common Stock
                                    suspended or delisted from trading on a
                                    Principal Market for in excess of five (5)
                                    Trading Days;

Then, or at any time thereafter, and in each and every such case, unless such
Event of Default shall have been waived in writing by the Holder (which waiver
shall not be deemed to be a waiver of any subsequent default) at the option of
the Holder and in the Holder's sole discretion, the Holder may consider this
Debenture immediately due and payable, without presentment, demand, protest or
notice of any kind, all of which are hereby expressly waived, anything herein or
in any note or other instruments contained to the contrary notwithstanding, and
the Holder may immediately enforce any and all of the Holder's rights and
remedies provided herein or any other rights or remedies afforded by law.

                  15.      Nothing contained in this Debenture shall be
construed as conferring upon the Holder the right to vote or to receive
dividends or to consent or receive notice as a shareholder in respect of any
meeting of shareholders or any rights whatsoever as a shareholder of the
Company, unless and to the extent converted in accordance with the terms hereof.

                  16.      In no event shall the Holder be permitted to convert
this Debenture for shares of Common Stock upon which, (x) the number of shares
of Common Stock owned by such Holder (other than shares of Common Stock issuable
upon conversion of this Debenture) plus (y) the number of shares of Common Stock
issuable upon conversion of this Debenture, would be equal to or exceed 9.9% of
the number of shares of Common Stock then issued and outstanding, including
shares issuable upon conversion of this


                                       6
<PAGE>

Debenture held by such Holder after application of this Paragraph 16. As used
herein, beneficial ownership shall be determined in accordance with Section
13(d) of the Securities Exchange Act of 1934, as amended, and the rules and
regulations thereunder. To the extent that the limitation contained in this
Paragraph 16 applies, the determination of whether this Debenture is convertible
(in relation to other securities owned by the Holder) and of which a portion of
this Debenture is convertible shall be in the sole discretion of such Holder,
and the submission of a Notice of Conversion shall be deemed to be such Holder's
determination of whether this Debenture is convertible (in relation to other
securities owned by such holder) and of which portion of this Debenture is
convertible, in each case subject to such aggregate percentage limitation, and
the Company shall have no obligation to verify or confirm the accuracy of such
determination. Nothing contained herein shall be deemed to restrict the right of
a holder to convert this Debenture into shares of Common Stock at such time as
such conversion will not violate the provisions of this Paragraph 16. The
provisions of this Paragraph 16 may be waived by the Holder of this Debenture
upon not less than 75 days' prior notice to the Company, and the provisions of
this Paragraph 16 shall continue to apply until such 75th day (or such later
date as may be specified in such notice of waiver). No conversion of this
Debenture in violation of this Paragraph 16 but otherwise in accordance with
this Debenture shall affect the status of the Common Stock issued upon such
conversion as validly issued, fully-paid and nonassessable. If instead of
receiving cash on the Maturity Date the Holder instead exercises its right to
convert this Debenture into Common Stock pursuant to Paragraph 5 by delivery of
a Notice of Conversion prior to receipt of payment, and such conversion would
cause the limit contained in the first sentence of this Paragraph 16 to be
exceeded, such conversion of this Debenture shall occur up to such limit and the
remaining unconverted portion of this Debenture shall be converted into Common
Stock (1) in accordance with one or more Notices of Conversion delivered by the
Holder or (2) 65 days after the Maturity Date, whichever is earlier.
Notwithstanding anything contained herein to the contrary, no interest shall
accrue after the Maturity Date on any such unconverted portion of this
Debenture.


                                       7
<PAGE>

                  IN WITNESS WHEREOF, the Company has caused this instrument to
be duly executed by an officer thereunto duly authorized.

Dated:   May __, 2000

                                         NHANCEMENT TECHNOLOGIES INC.

                                         By:
                                            -----------------------------------
                                         Name:
                                              ---------------------------------
                                         Title:
                                               --------------------------------

Attest:



-----------------------


                                       8
<PAGE>

                                    EXHIBIT A

                              NOTICE OF CONVERSION

   (To be Executed by the Registered Holder in order to Convert the Debenture)

                  The undersigned hereby irrevocably elects to convert
$ ________________ of the principal amount of the above Debenture No. ___ into
Shares of Common Stock of Nhancment Technologies, Inc. (the "Company") according
to the conditions hereof, as of the date written below.

Date of Conversion*
                    -----------------------------------------------------------

Conversion Price *
                   ------------------------------------------------------------

Accrued Interest
                 --------------------------------------------------------------

Signature
          ---------------------------------------------------------------------
                                                [Name]

Address:
        -----------------------------------------------------------------------

        -----------------------------------------------------------------------


*If such conversion represents the remaining principal balance of the Debenture,
the original Debenture must accompany this notice within three Trading Days.


                                       9